Refresh contributing guidelines to match current situation

[osma]

Reasons for creating this PR

The CONTRIBUTING.md document is quite old, from 2016. The development situation has changed since then so it needed a refresh.

Link to relevant issue(s), if any

• Closes Need for contributing guidelines document #1286

Description of the changes in this PR

Update sections of CONTRIBUTING.md to match current situation and link to most recent resources.

Note that I have also added a link to CONTRIBUTING.md to the home page of the wiki (last section about development processes), as suggested in #1286.

Known problems or uncertainties in this PR

n/a

Checklist

• phpUnit tests pass locally with my changes
• I have added tests that show that the new code works, or tests are not relevant for this PR (e.g. only HTML/CSS changes)
• The PR doesn't reduce accessibility of the front-end code (e.g. tab focus, scaling to different resolutions, use of .sr-only class, color contrast)
• The PR doesn't introduce unintended code changes (e.g. empty lines or useless reindentation)

[codecov]

Codecov Report

Merging #1359 (f91f139) into master (0640b5e) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1359   +/-   ##
=========================================
  Coverage     71.15%   71.15%           
  Complexity     1649     1649           
=========================================
  Files            32       32           
  Lines          3786     3786           
=========================================
  Hits           2694     2694           
  Misses         1092     1092           

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[kinow]

👍 huge improvement. A few random comments, but nothing that really needs to be incorporated into this change.

[osma]

Thanks a lot for the suggestions @kinow , I will try to incorporate some/all of them into this PR!

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
No Duplication information

[osma]

I made all the changes suggested by @kinow , thanks again! I think it's now good enough so I'm merging this.