Improve rendering of `.. todo` directive

[amotl]

Apparently, the .. todo directive renders itself with a bit too much of a left padding or margin. Maybe we can do something about it on a future iteration. It is not too important right now.

-- https://crate-docs-theme--378.org.readthedocs.build/en/378/rst/misc.html
-- https://crate-docs-theme--378.org.readthedocs.build/en/378/myst/misc.html

Originally posted by @amotl in #378 (comment)

[amotl]

I think this can be considered fixed. I think the current rendering is reasonable. Closing this.

-- https://crate-docs-theme.readthedocs.io/en/latest/rst/misc.html
-- https://crate-docs-theme.readthedocs.io/en/latest/myst/misc.html