docs: clarify pipeline serialization

[nightcityblade]

Description

Fixes #13642.

The saving and loading guide now presents nlp.to_disk() with spacy.load() as
the straightforward way to persist and restore a complete pipeline directory.
It retains the in-memory to_bytes() example and clarifies why that workflow
must preserve the pipeline config separately.

Tests:

• npm run lint
• npm run --ignore-scripts build after generating the website quickstart module
• Prettier check for the changed MDX section

Types of change

Documentation update.

Checklist

• I confirm that I have the right to submit this contribution under the project's MIT license.
• I ran the tests, and all new and existing tests passed.
• My changes don't require a change to the documentation, or if they do, I've added all required information.