Fix http links underscore

[y0z]

Motivation

In reStructuredText, one-off hyperlinks should be represented with two trailing underscores rather than a single trailing underscore that creates a reference.
However, in Optuna, some documents do not follow the rule.

• https://github.com/search?q=repo%3Aoptuna%2Foptuna+%60_&type=code

Suggestion

Replace _ with __ in proper contexts.

Reference

• https://stackoverflow.com/questions/27420317/restructured-text-rst-http-links-underscore-vs-use
• https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html#anonymous-hyperlinks

[47aamir]

trying it

[y0z]

Follow-up tasks remain.

#5515 (review)

Thank you for the PR, LGTM!

Followup (core developers will check if any lines are missing the same change)

• faq.rst
• terminator.py
• optuna/visualization/_edf.py
• optuna/visualization/matplotlib/_edf.py
• optuna/visualization/matplotlib/_intermediate_values.py