Migrate contributing contents to a separate documentation site.

[Ivorforce]

Important

We're discussing / working on moving the contributing information out of the engine documentation. If you want to discuss this change, please join the discussion on RocketChat. Please discuss only implementation details in this PR.

This PR is the tandem to the WIP repository for new contributions documentation.

Reasoning

Here's why we're considering making this change:

• We want to make it easier to get started contributing by giving it a dedicated space.
• The contribution guidelines change independently of engine versions. We especially don't want outdated info hanging around in old engine documentation.
• The contribution guidelines don't need to be translated because contributors (for better or worse) need to communicate in english.
• The engine documentation is alleviated from contributing information and guidelines, making it easier to navigate and search for engine related information.

Structure

Contributing is removed, but not everything in this section is relevant only to engine contributors. People making modules or GDExtensions also need to know about engine development, and this information correlates to specific engine versions. Therefore, significant chunks of the Contributing section should persist in this repository.

The engine information however needs a new home. I'm suggesting new section Engine details. This currently houses the 3 subsections for Architecture / Code, Development / Buildsystem / Tooling, and File Formats.
I think this new section will benefit the engine documentation, as it isn't buried so deep among contributing information. People looking to understand the source code without necessarily contributing should find this information much more easily.

[AThousandShips]

Everything removed here makes sense to me, will check the other repo as well

[mhilbrunner]

@Ivorforce This needs a rebase and update now to replace the URL with https://contributing.godotengine.org

[Ivorforce]

I rebased onto latest master (and replaced the url). There were a few merge conflicts:

All of them should be trivially resolvabe because the pages stay on this repo, they were just moved.
contributing/development/editor/creating_icons.rst is the exception, which I've synced to https://github.com/godotengine/godot-contributing-docs/blob/main/engine/editor/creating_icons.rst.

[Ivorforce]

Ok, I managed to fix all the compile problems on this PR.

Some changes of note:

• I changed problematic redirects.csv entries by simply removing the pages that were referenced.
• I brought back the class reference primer and two pages of the editor documentation, as they were very technical and engine version specific. The contribution docs keep the editor style guide and contributing to the class reference pages as those are more contributions workflow focused. I'm pretty happy with the new split.
• I needed to edit some class references. The changes are reflected in a parallel godot engine PR.
• The rest of the links were hardcoded to the new docs (i.e. https://contributing.godotengine.org/en/latest/...).

Note that the new docs still need to be finalized for this PR to be mergable.

[mhilbrunner]

cc @AThousandShips @skyace65 A final review pass would be welcome if anyone wants to

[skyace65]

Will look this over sometime this weekend

[AThousandShips]

I'll take a look next week

[skyace65]

I built this branch and didn't get any errors so this looks good to me. We need to remember to move over issues relating to the contributing section to the new repository when this is merged.