Improve doc theme and update README

[bruhnild]

Description

Related Issue

Checklist

• I have followed the guidelines in our Contributing document
• My code respects the Definition of done available in the Development section of the documentation
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes
• I added an entry in the changelog file
• My commits are all using prefix convention (emoji + tag name) and references associated issues
• I added a label to the PR corresponding to the perimeter of my contribution
• The title of my PR mentionned the issue associated

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.46%. Comparing base (4c33d45) to head (ef41df0).
Report is 14 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4408   +/-   ##
=======================================
  Coverage   98.46%   98.46%           
=======================================
  Files         268      268           
  Lines       21290    21290           
=======================================
  Hits        20964    20964           
  Misses        326      326           

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[cypress]

Geotrek-admin    Run #10577

Run Properties:   Passed #10577  •  8c4986361a ℹ️: Merge ef41df034a6fa3c4041ae7a09f9f089be7ec8526 into 875baeaea754c326376f00e9dd7e...

Project	Geotrek-admin
Branch Review	refs/pull/4408/merge
Run status	Passed #10577
Run duration	02m 06s
Commit	8c4986361a ℹ️: Merge ef41df034a6fa3c4041ae7a09f9f089be7ec8526 into 875baeaea754c326376f00e9dd7e...
Committer	Marine Faucher
View all properties for this run ↗︎

---

Test results
Failures	0
Flaky	0
Pending	0
Skipped	0
Passing	22
View all changes introduced in this branch ↗︎

[camillemonchicourt]

Je passerai bien toute la doc de RST à MD partout.
Par contre commencer à mettre du HTML, je suis moins chaud, mais si c'est utile, pourquoi pas.

[bruhnild]

Je passerai bien toute la doc de RST à MD partout. Par contre commencer à mettre du HTML, je suis moins chaud, mais si c'est utile, pourquoi pas.

J'ai effectué le remplacement du fichier README.rst par un fichier README.md

L'objectif de cette PR est d'harmoniser les readme entre les briques de Geotrek dans un souci de cohérence. J'ai donc repris la même structure que sur GTR ou GTW . Le HTML est utile pour la mise en page des titres et des badges.