docs: replace inline ASGI server list with link to canonical implemen…

[EmmanuelNiyonshuti]

Summary

The installation section was listing ASGI servers inline (uvicorn, daphne, hypercorn), which is incomplete and will go stale over time.

Replaced the inline list with a link to the canonical, up-to-date list of server implementations at https://asgi.readthedocs.io/en/latest/implementations.html#servers. #3203

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change. (N/A, docs-only change)
• I've updated the documentation accordingly.

[EmmanuelNiyonshuti]

Also noticed the same line in README.md, included that in this PR too.

[EmmanuelNiyonshuti]

Anycorn is in now django/asgiref#557 this should be good to go I think?, let me know if something else is needed.

[cubic-dev-ai]

No issues found across 2 files

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