Docs : added guidance on what to do after the installation

[omiingale-oi]

Description

Adds a short Next steps section to installation.rst. After installing libpysal , new users have no clear guidance on where to go next in the documentation.
The added text point users to the User Guide introduction, where they can see examples and understand the main parts of the library.

This improves the beginner experience without changing any functionallity.

Checklist

• You have run tests on this submission locally using pytest on your changes.
  N/A: This is a documentation only change (does not affect code/tests).

• This pull request is directed to the pysal/main branch.

• This pull introduces new functionality covered by docstrings and unittests.
  N/A: No new functionality/code changes are introduced.

• You have assigned a reviewer and added relevant labels.
  N/A: No reviewer assigned, no labels added.

• The justification for this PR is:
  After installing libpysal , new users are not given any guidance on what to read or try next. This PR adds a “Next steps” section to point them to the User Guide, making the documentation flow clean & clear for beginners.

No AI tools were used in this contribution.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.5%. Comparing base (7304239) to head (14c9835).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #864     +/-   ##
=======================================
+ Coverage   85.5%   85.5%   +0.1%     
=======================================
  Files        151     152      +1     
  Lines      16092   16114     +22     
=======================================
+ Hits       13758   13785     +27     
+ Misses      2334    2329      -5     

Files with missing lines	Coverage Δ
libpysal/cg/tests/test_kdtree.py	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

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