MAINT: Fix CIs

[larsoner]

1. Use .. include to deduplicate index.rst and README.rst
2. Fix spelling error
3. Remove tight_layout that is no longer needed (and emits a warning)

[codecov]

Codecov Report

Merging #525 (d72b562) into main (047a27a) will decrease coverage by 1.54%.
The diff coverage is 100.00%.

❗ Current head d72b562 differs from pull request most recent head 32d0854. Consider uploading reports for the commit 32d0854 to get more accurate results

@@            Coverage Diff             @@
##             main     #525      +/-   ##
==========================================
- Coverage   95.22%   93.68%   -1.54%     
==========================================
  Files          69       69              
  Lines        2868     2867       -1     
  Branches      412      412              
==========================================
- Hits         2731     2686      -45     
- Misses         66      112      +46     
+ Partials       71       69       -2     

Files	Coverage Δ
mne_nirs/conftest.py	96.49% <ø> (ø)
mne_nirs/visualisation/_plot_quality_metrics.py	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes