Fix documentation build for nbconvert 7.16.6 upgrade

[Copilot]

Type

• Bug fix (non-breaking change which fixes an issue)

Motivation and Context

The nbconvert 6.5.4 → 7.16.6 upgrade introduced two dependency conflicts preventing documentation builds:

1. bleach[css] (required by nbconvert 7.x) needs tinycss2 but wasn't declared
2. m2r2 requires mistune==0.8.4 while nbconvert 7.x requires mistune>=2.0.3,<4

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

Dependencies (docs/requirements.txt):

• Add tinycss2>=1.1.0,<1.5 (required by bleach[css])
• Replace m2r2==0.3.3.post2 with myst-parser==2.0.0 (actively maintained, compatible with mistune 2.x+)

Sphinx configuration (docs/conf.py):

• Replace m2r2 extension with myst_parser

RST includes (docs/introduction.rst, docs/tutorial/visualization/tensorboard_plugin.rst):

• Migrate from .. mdinclude:: to .. include:: with :parser: myst_parser.sphinx_ directive

myst-parser is the modern successor to m2r2 for Markdown→RST conversion in Sphinx.

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[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.

[ssheorey]

@copilot go ahead and start.

[Copilot]

@copilot go ahead and start.

The dependency fixes are complete in commit 5a7fad9. The documentation build should pass once the workflow is approved and runs. All dependency conflicts have been resolved:

• Added tinycss2 for bleach[css]
• Replaced m2r2 with myst-parser to fix mistune version conflict