Link attention docs to the main docs and fix errors reported by Sphinx

[ptrendx]

Description

Linked the attention docs to the html docs, fixed errors reported by Sphinx (cyclical imports, wrong indentation, wrong section names). Additionally tried to make the attention docs render nicer in HTML (turns out nbsphinx does not support many things that were used in that notebook, like <br> inside the table)

Type of change

• [x] Documentation change (change only to the documentation, either a fix or a new content)
• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Infra/Build change
• [ ] Code refractor

Changes

Please list the changes introduced in this PR:

• Added attention notebook to docs/index.rst
• Made changes to some tables in the attention notebook to render better in HTML (although still not great).
• Fixed the errors reported by Sphinx
• Reverted the Sphinx version on GitHub to be consistent with the one in the internal CI - the new Sphinx version introduces multiple rendering issues. Will try to fix them in a followup.

Checklist:

• [x] I have read and followed the contributing guidelines
• [x] The functionality is complete
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes

[ptrendx]

/te-ci