Various improvements to documentation

[arbimo]

• fix(doc): Fix broken link to compiler notebook.
• doc: Add contributor's guide
• fix(doc): Fix syntax error in markdown source (CONTRIBUTING.md)
• doc: Initial work on improved engine pages

[codecov-commenter]

Codecov Report

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

Comparison is base (282488f) 85.08% compared to head (faf34e8) 85.08%. Report is 16 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #517 +/- ## ======================================= Coverage 85.08% 85.08% ======================================= Files 200 200 Lines 26336 26336 ======================================= Hits 22407 22407 Misses 3929 3929 ```

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[arbimo]

This PR refactors some parts of the documentation as a preparation for the final deliverable. It would be great to have it merged quickly so I can then ping other people to fill it more.

Notable changes:

• Add "Application Showcase" section (for use-case inspired guidelines), currently with space and drones use-cases.
• Initial work on presenting the available engines in a table with high-level features
• "Problem Representation" page should act as an overview of the various problem classes with pointers to detailed presentations (done for hierarchical and scheduling)
• Added a "Contributor's guide" page, currently with explanation to build and update the documentation.
• Added a "What is Planning" page
• Integrated ICAPS tutorial material (currently some overlap with the Quickstart)

@Framba-Luca could you fill in the docs/engines/04_testing_engine.rst file with documentation on how to run the report tool and what is expected for a valid integration.

@mikand: The "Planning Engine Examples" (docs/notebooks/engines/README.md) page was needed for a deliverable right? It feel a bit strange where it currently is or could even be removed entirely.

@alvalentini Could you review the docs/showcases/01-space.md document and perhaps add an illustration.

[alvalentini]

@arbimo The Planning Engines Example page is not needed in the ReadTheDocs documentation. We've included this link (https://github.com/aiplan4eu/unified-planning/tree/master/docs/notebooks/engines) in a deliverable and we only need to keep the file reachable through that link.

[arbimo]

Thanks @alvalentini and @Framba-Luca I have made a few changes following your remarks and contributions. I think this is now ready. I will ping engine developers to validate and complete it once the documentation is easily visible.