Documentation: getting started to installation

[RemDelaporteMathurin]

Proposed changes

I noticed that in the docs there was a confusion between "Getting started", "User guide: begginer's guide", "Installation" etc.

So I renamed "Getting started" by "Installation"

Types of changes

What types of changes does your code introduce to FESTIM?

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code refactoring
• Documentation Update (if none of the other choices apply)
• New tests

Checklist

• Black formatted
• Unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4500c9c) 98.95% compared to head (3caa4ce) 98.95%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #687   +/-   ##
=======================================
  Coverage   98.95%   98.95%           
=======================================
  Files          58       58           
  Lines        2195     2195           
=======================================
  Hits         2172     2172           
  Misses         23       23           

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[jhdark]

Improves clarity! sounds good to me