Add the C, CPP, JavaRS, KotlinC, KotlinSync, PHP, Ruby Getting Started Code

[tmcneil-mdb]

Description

Adds the remaining six Get Started sample applications — C, C++, Java Reactive
Streams, Kotlin Coroutines, Kotlin Sync, PHP, and Ruby — mirroring the existing
go/hello-world, node/hello-world, python/hello-world, dotnet/hello-world,
and java/hello-world samples. Each app connects to a MongoDB deployment using
the connection string in the MONGODB_URI environment variable, seeds a few
sample product documents into the get_started.products collection, then
queries and prints one of them.

• c/hello-world/ — sample app, CMake build (mongoc 2.0), README, .gitignore
• cpp/hello-world/ — sample app, CMake build (mongocxx 4.0), README, .gitignore
• java-rs/hello-world/ — sample app, Maven build (mongodb-driver-reactivestreams via mongodb-driver-bom 5.9.1, Project Reactor 2025.0.0, Java 21), README, .gitignore
• kotlin-coroutine/hello-world/ — sample app, Maven build (mongodb-driver-kotlin-coroutine 5.9.1, Kotlin 2.1.0), README, .gitignore
• kotlin-sync/hello-world/ — sample app, Maven build (mongodb-driver-kotlin-sync 5.9.1, Kotlin 2.1.0), README, .gitignore
• php/hello-world/ — sample app, Composer manifest (mongodb/mongodb 2.0, ext-mongodb 2.0), README, .gitignore
• ruby/hello-world/ — sample app, Gemfile (mongo 2.0), README, .gitignore
• Added the C, C++, Java Reactive Streams, Kotlin Coroutines, Kotlin Sync, PHP,
  and Ruby entries to the root README

This supersedes the following PRs, which targeted the wrong repository:

• DOCSP-61799: C Getting Started mongodb-code-examples#6 (C)
• DOCSP-61800: CPP Getting Started mongodb-code-examples#7 (C++)
• DOCSP-61803: JavaRS Getting Started mongodb-code-examples#8 (Java Reactive Streams)
• DOCSP-61805: Kotlin Coroutine Getting Started mongodb-code-examples#9 (Kotlin Coroutines)
• DOCSP-61806: Kotlin Sync Getting Started mongodb-code-examples#10 (Kotlin Sync)
• DOSCP-61807: PHP Getting Started mongodb-code-examples#11 (PHP)
• DOCSP-61809: Ruby Getting Started mongodb-code-examples#12 (Ruby)

README changes

• Added Windows commands throughout, following the powershell convention
  established by python/hello-world/README.md
• Noted in the Ruby README that the driver is not officially supported on
  Windows, and omitted Windows commands there.

Testing

All seven samples were run end to end on macOS by following the documented
setup steps in each README, connecting to an Atlas deployment and confirming
the expected product document was printed.

JIRA

• DOCSP-61799 — C
• DOCSP-61800 — C++
• DOCSP-61803 — Java Reactive Streams
• DOCSP-61805 — Kotlin Coroutines
• DOCSP-61806 — Kotlin Sync
• DOCSP-61807 — PHP
• DOCSP-61809 — Ruby