DOC: change rocketpy landing page to standard code docs.

[phmbressan]

Pull request type

• [X] ReadMe, Docs and GitHub updates

Checklist

• [x] CHANGELOG.md has been updated (if relevant)

Current behavior

The current landing page of RocketPy documentation, despite being unique and self updating with the README, does not have the most friendly and straight to the point interface. The issues it entails are:

• The page has a rather long scrollable that mirrors our README, not all users seek reading it every time;
• In spite of having these sections in the top nav, newcomers might have trouble figuring out which page of the docs to go.

New behavior

This PR aims to change our landing page to a layout more familiar and similar to what is used by many other code based repositories. A summary of the changes:

• The long README was removed from the landing page to favor more direct buttons;
• Each page/"section" of RocketPy docs is clearly linked and their purpose explained;

Preview of the Changes

Click to see a preview of the new page:

Breaking change

• [ ] Yes
• [X] No

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 73.62%. Comparing base (73ec844) to head (1be4b29). Report is 14 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #584      +/-   ##
===========================================
+ Coverage    73.56%   73.62%   +0.05%     
===========================================
  Files           70       70              
  Lines        10313    10313              
===========================================
+ Hits          7587     7593       +6     
+ Misses        2726     2720       -6     

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