[REL] remove dev in sphinx directives related to version

[Remi-Gau]

• Closes none

Changes proposed in this pull request:

• update sphinx directives to use X.Y.Z format in view of next release
• update maintenance documentation

[github-actions]

👋 @Remi-Gau Thanks for creating a PR!

Until this PR is ready for review, you can include the [WIP] tag in its title, or leave it as a github draft.

Please make sure it is compliant with our contributing guidelines. In particular, be sure it checks the boxes listed below.

• PR has an interpretable title.
• PR links to Github issue with mention Closes #XXXX (see our documentation on PR structure)
• Code is PEP8-compliant (see our documentation on coding style)
• Changelog or what's new entry in doc/changes/latest.rst (see our documentation on PR structure)

For new features:

• There is at least one unit test per new function / class (see our documentation on testing)
• The new feature is demoed in at least one relevant example.

For bug fixes:

• There is at least one test that would fail under the original bug conditions.

We will review it as quick as possible, feel free to ping us with questions if needed.

[codecov]

Codecov Report

Merging #4013 (f9135e4) into main (b214dce) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #4013   +/-   ##
=======================================
  Coverage   91.77%   91.77%           
=======================================
  Files         134      134           
  Lines       15795    15795           
  Branches     3299     3299           
=======================================
  Hits        14496    14496           
  Misses        755      755           
  Partials      544      544           

Flag	Coverage Δ
macos-latest_3.10	91.69% <ø> (ø)
macos-latest_3.11	91.69% <ø> (ø)
macos-latest_3.8	91.65% <ø> (ø)
macos-latest_3.9	91.65% <ø> (ø)
ubuntu-latest_3.10	?
ubuntu-latest_3.11	91.69% <ø> (ø)
ubuntu-latest_3.8	91.65% <ø> (ø)
ubuntu-latest_3.9	91.65% <ø> (ø)
windows-latest_3.10	91.63% <ø> (ø)
windows-latest_3.11	91.63% <ø> (ø)
windows-latest_3.8	91.59% <ø> (ø)
windows-latest_3.9	91.60% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
nilearn/_utils/docs.py	91.86% <ø> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more