Upgrade documentation techstack

[saurabheights]

Type

• Bug fix (non-breaking change which fixes an issue): Fixes #
• New feature (non-breaking change which adds functionality). Resolves #
• Breaking change (fix or feature that would cause existing functionality to not work as expected) Resolves #

Motivation and Context

Focusing on upgrading documentation dependencies and removing dependency on open3d_sphinx_theme.

The changes done in open3d_sphinx_theme can be overridden without needing to fork another repository. This allows easier upgrade of documentation tech stack in future.

I only incorporated one change from
readthedocs/sphinx_rtd_theme@master...isl-org:open3d_sphinx_theme:master which is to support selection of different version of documentation.

@ssheorey Search is working.

WIP - This change breaks layout of references. Badly too. Almost fixed.

Checklist:

• I have run python util/check_style.py --apply to apply Open3D code style
  to my code.
• This PR changes Open3D behavior or adds new functionality.
  • Both C++ (Doxygen) and Python (Sphinx / Google style) documentation is
    updated accordingly.
  • I have added or updated C++ and / or Python unit tests OR included test
    results (e.g. screenshots or numbers) here.
• I will follow up and update the code if CI fails.
• For fork PRs, I have selected Allow edits from maintainers.

Description

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This change is 

[saurabheights]

Another Issue - There is a lot of extra vertical margin added between elements in docs.

[saurabheights]

JUST FYI - Replacing sphinx-rtd-theme with furo, which provides dark and light themes along with using system theme. Also, references works nicely.

References works well with both furo and sphinx-rtd-theme, only when docutils version <= 0.17.0.

pradyunsg/furo#729
readthedocs/sphinx_rtd_theme#1540

[ssheorey]

Hi @saurabheights thanks for taking this up! The new theme looks more modern. Let me know when it's ready for review.

[ssheorey]

Looks great! Go ahead and switch to the furo theme if you like. Thanks!

[saurabheights]

@ssheorey Just an update on progress - furo theme works well until docutils version 0.17.0, so I upraded packages until this version. I have added minor css change for toctree captions. The references are slightly incorrect but we can ignore the issue for now as it is minor (I will try to find its fix as well). The main pending blocker is pradyunsg/furo#500 which provides support for multiple version. I am in process of overriding the theme to inject the html and css from http://www.open3d.org/docs/versions.js.

A solid upvote for using furo is the right navigation bar which shows TOC of webpage making navigating and search much easier for large classes. (Ignore bottom left, still working on navigation bar).

P.S. I am travelling international, so will be busy for next week. Please let me know if this PR is priority, say for next version release. I will try my best to wrap it up sooner.

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[saurabheights]

Wanted to highlight that since furo is center-aligned theme, unlike rtd, its better to go for center alignment for images. (IMO, although correct me if you have a contradicting thoughts).

[saurabheights]

@ssheorey Ready for review. Check each commit message to see summary. The reference page doesnt render well but I would prefer to wait for an upstream-fix. Lemme know if thats a blocker for merge.

Navigation panels are working well, although dont look great.

P.S. Side request - Please will it be possible to get some feedback on #6446. You can also reach out to me on discord.