Fixed Installation Reference Bad Link Issue #6732

[error9098x]

**What is this PR

Closes #6732
The issue was caused by the ref as the same ref name is used in the same page ( https://www.pymc.io/projects/docs/en/latest/learn/core_notebooks/pymc_overview.html#installation ) , #installation was causing the issue, so I changed the ref name to #installationguide and also changed installation.md to installation.rst as markdown was somehow not working to reference back to installation page.

Now, after fixing, it leads to the correct link with {ref}Installation Guide
**
Do let me know if any changes are required :)
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Checklist

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• Link relevant issues (preferably in nice commit messages)
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[OriolAbril]

sorry, I did not see this PR. The issue is actually the inadvertent use of "sphinx.ext.autosectionlabel" which was creating an installation target (because there is a section called installation) which collided with the proper installation target in installation.md. It will be fixed by #6781 among other things. The fix to this specific issue though is removing this extension from the extension list.