Update docs.yml

[victordibia]

Bump python version, pin nr-utils

Fix docs build CI bug.

• pydocmkardown depends on pydocspec
  - pydocspec was updated May 6 (two days ago), and includes a problematic dependency (nr.utils which is specified as nr-utils. This caused imports to fail.
• current fix is fixing the pydocspec version.
• Also using the opportunity to update the python version 3.8 -> 3.9 for the docs build 0.2

Why are these changes needed?

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.56%. Comparing base (b17793e) to head (eb7b490).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6493   +/-   ##
=======================================
  Coverage   78.56%   78.56%           
=======================================
  Files         225      225           
  Lines       16549    16549           
=======================================
  Hits        13001    13001           
  Misses       3548     3548           

Flag	Coverage Δ
unittests	78.56% <ø> (ø)

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