[FLINK-40282][docs] Expand and correct the concepts Glossary

[nicusX]

What is the purpose of the change

The current Flink Glossary doc page has gaps, some inconsistencies, and ambiguities which may be confusing for new Flink users. It also focus on DataStream API exclusively.
This PR expands and fixes the Glossary page.

Brief change log

New entries (10)

• Checkpoint, Savepoint: referenced by other entries but never defined
• Watermark, Parallelism, Channel, Source Split: absent
• Flink SQL Statement, Statement Set, StreamExchange Operator: first Table API / SQL
  coverage in the glossary
• Job Graph: promoted from an alias to a full entry
• Added 2 pointer entries: StreamGraphs --> Logical Graph, Execution Graph --> Physical Graph

Corrected definitions

• Job Graph no longer documented as a synonym of Logical Graph
• Task and Sub-Task no longer conflated
• Physical Graph fixed definition
• Partition split into Logical Partitioning and Physical Partitioning sub-entries; also added note about for external system such as Kafka topic partitions
• State Backend reframed as the storage itself rather than a configuration
• Checkpoint Storage adds the durable-storage framing and a pointer to disaggregated state
• (Runtime) Execution Mode was a bare link to another page; now describes STREAMING and
  BATCH modes
• Flink Application: Java or Python, DataStream or Table API (was "a Java Application").
• Fixed 3 broken cross-links that used capitalized anchors which Hugo doesn't generate (#Record and #Function).

Expanded entries

Flink Job, Logical Graph, Operator (adds Source/Sink Operators and a note that the docs use
"Operator" loosely for Task/Sub-Task), Operator Chain, Function (Python, UDF, PTF), Flink Cluster,
Flink JobManager, Flink JobMaster, Flink TaskManager, Sub-Task

Consistency

• Applied one capitalization rule: a defined term is capitalized when it refers to a glossary entry
  and lowercase in generic English use.
• Restored alphabetical order, disregarding the Flink prefix (5 entries were misplaced). The page
  sets bookToc: false, so ordering is the only way to locate a term.
• Retitled for consistency with sibling entries: ExecutionGraph --> Execution Graph,
  JobGraph --> Job Graph, UID hash --> UID Hash
• Anchors unchanged except #jobgraph --> #job-graph, which is not referenced outside of this page
• Added "Also " lines recording the spellings readers meet in code, logs and the Web UI
• Removed trailing whitespace.

Verifying this change

• Docs changes only

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): No
• The public API, i.e., is any changed class annotated with @Public(Evolving): No
• The serializers: No
• The runtime per-record code paths (performance sensitive): No
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: No
• The S3 file system connector: No

Documentation

• Does this pull request introduce a new feature? No
• If yes, how is the feature documented? not applicable

---

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

[flinkbot]

CI report:

• 9b08303 Azure: SUCCESS
• 6386b49 Azure: PENDING
• 78e3fb3 Azure: PENDING

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[alpinegizmo]

There are a few points where I either disagree, or think some additional clarification is needed.

[nicusX]

Thanks for reviewing this, @alpinegizmo
I should have addressed all the comments

[nicusX]

I split entry reordering into a separate FLINK-40383 and PR #28969. To simplify reviewing this PR once the other is merged

[alpinegizmo]

I think this is solid. If you wanted to go further, I think we could find more terms to add -- the one that comes to mind immediately is key group -- but I think this is would be good place to stop.

[davidradl]

I am not sure what " most Transformations are implemented by certain Operators" is telling me. I would either remove this sentence or be explicit about the certain operators you are referring to or point to how to find the details.

[nicusX]

I agree the sentence is unclear.
I think it's important to keep it because users think Tranformations == Operators, which is not true.
I expanded the wording and added examples from DataStream API (Table API/SQL would bring us down a rabbithole).
@davidradl please let me know if it's clear now

[davidradl]

should we mention allowed lateness here?

[nicusX]

@davidradl I think this would be too much for a Glossary. The Watermark section is already very long.
I deliberately decided to stop at this point because Watermarks would deserve a page (a book?) by themselves :)
In particular, mentioning allowed lateness would require: 1) explaining why some operators but not others drop late events, 2) how this behaviour is modified by allowed lateness, 3) how these operators behave on receiving late events within allowed lateness, 4) how this affect state retention, and that 5) allowed lateness is only available in some operators and DataStream API only....

[nicusX]

@alpinegizmo I added Key Group, as you suggested.
I deliberately omitted saying that this why changing maxParallelism breaks the ability to restore from a snapshot. It looked too much for a Glossary