docs(deps): pin sphinx_rtd_theme to >=0.6 (#1615)

See
readthedocs/sphinx_rtd_theme#1571 (comment)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Chores**
- Updated the version constraint for the `sphinx_rtd_theme` package to
ensure compatibility, enhancing project stability and performance.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: Jinzhe Zeng <[email protected]>

doc/requirements.txt

@@ -1,7 +1,7 @@
 # https://github.com/sphinx-doc/sphinx/issues/11662
 sphinx>=4.0.2,!=7.2.5
 recommonmark
-sphinx_rtd_theme
+sphinx_rtd_theme>=0.6
 sphinx_markdown_tables
 sphinx-argparse<0.5.0
 myst-parser