Fix broken RST links in README and update outdated docs

[veeceey]

What do these changes do?

Fix several documentation issues across README.rst and docs/index.rst:

1. README.rst: Fix two malformed RST hyperlinks for "Signals" and "Web Server Advanced" -- both were missing the closing >`_ syntax, causing them to render as broken text on PyPI and GitHub instead of proper clickable links.

2. docs/index.rst: Add missing await keyword in the type-checker code example. Signal.send() is an async method (the docs even mark it with :async:), but the example showed signal.send(42, "foo") instead of await signal.send(42, "foo").

3. docs/index.rst: Update Python version requirement from 3.8 to 3.9 in two places. Python 3.8 support was dropped in v1.3.2, and setup.cfg already specifies python_requires = >=3.9.

4. docs/index.rst: Update frozenlist dependency version from >= 1.0.0 to >= 1.1.0 to match what setup.cfg actually declares.

Are there changes in behavior for the user?

No behavior changes. Documentation-only fixes.

Related issue number

None -- found by code inspection.

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
• Add a new news fragment into the CHANGES folder

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.34%. Comparing base (85431dd) to head (bc1d37a).
⚠️ Report is 35 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #794   +/-   ##
=======================================
  Coverage   99.34%   99.34%           
=======================================
  Files           2        2           
  Lines         153      153           
  Branches        6        6           
=======================================
  Hits          152      152           
  Misses          1        1           

Flag	Coverage Δ
CI-GHA	99.34% <ø> (ø)
MyPy	91.33% <ø> (ø)
OS-Linux	99.34% <ø> (ø)
Py-3.10.18	?
Py-3.10.19	95.91% <ø> (?)
Py-3.11.13	?
Py-3.11.14	95.91% <ø> (?)
Py-3.12.11	?
Py-3.12.12	95.91% <ø> (?)
Py-3.13.12	99.34% <ø> (?)
Py-3.13.5	?
Py-3.9.23	?
Py-3.9.25	95.91% <ø> (?)
VM-ubuntu-24.04	97.95% <ø> (ø)
VM-ubuntu-latest	91.33% <ø> (ø)
pytest	97.95% <ø> (ø)

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