Starting to get rid of {eval-rst} blocks on documentation files

[junietoc]

Small fix on #1197

I have detected the presence of {eval-rst} blocks in the following files.

• api.md
• background.md
• changelog.md
• gallery.md
• index.md
• quickstart.md

I started making fixes in some of these files.
Your feedback on this PR would be greatly appreciated

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📚 Documentation preview 📚: https://poliastro--1633.org.readthedocs.build/en/1633/

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.04 🎉

Comparison is base (9d9e024) 91.05% compared to head (76faf01) 91.10%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1633      +/-   ##
==========================================
+ Coverage   91.05%   91.10%   +0.04%     
==========================================
  Files         116      116              
  Lines        4708     4708              
  Branches      352      352              
==========================================
+ Hits         4287     4289       +2     
+ Misses        334      333       -1     
+ Partials       87       86       -1     

Impacted Files	Coverage Δ
src/poliastro/maneuver.py	100.00% <100.00%> (+2.27%)	⬆️

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