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 We especially don't want outdated info hanging around in old engine documentation.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