docs: fix keywords metadata

[Revathyvenugopal162]

Description

keywords in the metadata should be comma seperated, https://docutils.sourceforge.io/docs/ref/rst/directives.html#meta
otherwise, all the keywords treated as single word by seo and give result

Issue linked

no issue created

Checklist

• I have visited and read Develop PyMAPDL.
• I have tested my changes locally
• I have added necessary documentation or updated existing documentation.
• I have followed the PyMAPDL coding style guidelines.
• I have added appropriate unit tests that also ensure the minimal coverage criteria.
• I have reviewed my changes before submitting this pull request.
• I have linked the issue or issues that are solved to the PR if any.
• I have assigned this PR to myself.
• I have marked this PR as draft if it is not ready to be reviewed yet.
• I have made sure that the title of my PR follows Commit naming conventions (e.g. feat: adding new MAPDL command)

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.09%. Comparing base (8573285) to head (a113d9b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3396      +/-   ##
==========================================
+ Coverage   85.07%   85.09%   +0.02%     
==========================================
  Files          55       55              
  Lines        9926     9926              
==========================================
+ Hits         8445     8447       +2     
+ Misses       1481     1479       -2     

[germa89]

thank you for this @Revathyvenugopal162 !! :)