Update ASGI instrumentation docs

[florimondmanca]

Description

Hola! 👋

Motivation

I was working through adding the ASGI opentelemetry-python instrumentation to awesome-asgi (see florimondmanca/awesome-asgi#47), and it popped out to me that the docs page for this instrumentation was a bit sub-par compared to what we have for most other instrumentations. That is, there's currently only an API reference, without usage examples.

Turns out we do have usage examples in the opentelemetry-instrumentation-asgi README, so…

Description of changes

I took the same approach than the Starlette instrumentation docs and updated the ASGI docs to:

• Include the ASGI instrumentation README.rst.
• List the existing API reference below.

I also:

• Added a "raw ASGI" usage example at the bottom of the page.
• Updated the page title to match the "XYZ Instrumentation" model for other pages.

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Built the docs locally and here's what the page now looks like:

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ Florimond Manca (3e9a9e2)

[codeboten]

Thanks for the contribution!