Refactor : Using stable documentation link instead of latest link.

[ambiguousphoton]

Using stable link instead of latest link in -> mslib/index.py -> content variable

Fixes #
Changed the URL to "https://mss.readthedocs.io/en/stable/gallery/index.html\"

Additional information for reviewer? :
This is pull request is to fix issue 2047 -> "#2047"

Does this PR results in some Documentation changes?
No

Checklist:

• [O] Bug fix. Fixes #<2047>
• New feature (Non-API breaking changes that adds functionality)
• [O] PR Title follows the convention of <type>: <subject>
• Commit has unit tests

[ReimarBauer]

Hi @ambiguousphoton
because this is a bug fix and it should go in the next patch level release, could you send it to stable?
Only this change. Develop will become the next major Release.