[DOC] reduce font size for headings

[Remi-Gau]

• Closes text overflow in documentation html rendering #3928

Technically this does not quite solve the overflow but indirectly helps by making the headings smaller.

This was especially needed on smaller viewports.

Changes proposed in this pull request:

• reduce headings size for different view ports

[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.

[Remi-Gau]

I suspect there may be a way to also avoid things like this 👍🏾

by removing the prefix in the nilearn.glm in the local table of content but I think this may require playing with jinja templating and arcane sphinx things that are not worth the effort

[codecov]

Codecov Report

Merging #3978 (153b8a5) into main (d1fadd4) will decrease coverage by 0.03%.
Report is 1 commits behind head on main.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3978      +/-   ##
==========================================
- Coverage   91.79%   91.76%   -0.03%     
==========================================
  Files         134      134              
  Lines       15776    15781       +5     
  Branches     3286     3289       +3     
==========================================
  Hits        14482    14482              
- Misses        751      755       +4     
- Partials      543      544       +1     

Flag	Coverage Δ
macos-latest_3.10	91.67% <ø> (-0.03%)	⬇️
macos-latest_3.11	91.67% <ø> (-0.03%)	⬇️
macos-latest_3.8	91.64% <ø> (-0.03%)	⬇️
macos-latest_3.9	91.64% <ø> (-0.03%)	⬇️
ubuntu-latest_3.10	91.67% <ø> (-0.03%)	⬇️
ubuntu-latest_3.11	91.67% <ø> (-0.03%)	⬇️
ubuntu-latest_3.8	91.64% <ø> (-0.03%)	⬇️
ubuntu-latest_3.9	91.64% <ø> (-0.03%)	⬇️
windows-latest_3.10	91.62% <ø> (?)
windows-latest_3.11	91.62% <ø> (-0.03%)	⬇️
windows-latest_3.8	91.58% <ø> (-0.03%)	⬇️
windows-latest_3.9	91.58% <ø> (-0.03%)	⬇️

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

see 1 file with indirect coverage changes

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

[Remi-Gau]

Fullwidth (1920px):

Before

After

[Remi-Gau]

Width 1090px

Before

After

[Remi-Gau]

width 390px

Before

After

[ymzayek]

LGTM