docs: improve gruf instrumentation documentation

[allexistence]

Fixes #2201

What this PR does

Improves gruf instrumentation documentation by:

• Replacing placeholder description
• Adding overview and "how it works"
• Documenting key source files

Notes

Doc only change

[arielvalentin]

Thanks for your contribution. This is a great start.

I'd recommend also explicitly documenting any instrumentation specific configuration options as well as any extensions to semantic conventions that are unique to this instrumentation.

Would you be able to review the source for config options and semantic attributes and document them?

[kaylareopelle]

Something else that's often helpful in READMEs is to document the available configuration options for a repository.

Here are gruf's configs:

opentelemetry-ruby-contrib/instrumentation/gruf/lib/opentelemetry/instrumentation/gruf/instrumentation.rb

Lines 16 to 20 in ac0c3c6

	option :peer_service, default: nil, validate: :string
	option :grpc_ignore_methods_on_client, default: [], validate: :array
	option :grpc_ignore_methods_on_server, default: [], validate: :array
	option :allowed_metadata_headers, default: [], validate: :array

We don't have a consistent approach for documenting configs. Here are a few examples:

• net_ldap
• redis
• grape
• sidekiq - This one is very different, as it just uses RDoc/Yard to document rather than including the content in the README. However, the content is very thorough.

[kaylareopelle]

Hi @allexistence. Thanks for your patience. I think your changes look good. There are failing checks now related to formatting. Can you take a look?

[thompson-tomo]

@allexistence can you add documentation to the top of the example controller as mentioned in https://github.com/open-telemetry/opentelemetry-ruby-contrib/actions/runs/28316231103/job/83890038917

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: allexistence / name: rishabh (a3aa982, be31edb)
• ✅ login: allexistence / name: rishank69@gmail.com (61b427d, 69b5e7e, 7c1c27e, 8663efb, 9c71efd, f4d93f0)
• ✅ login: kaylareopelle / name: Kayla Reopelle (cd93eb5)
• ✅ login: thompson-tomo / name: James Thompson (73f85bd, 977dd84)

[thompson-tomo]

Looks good, failing tests are unrelated.