[CHORE] Commit start of a Developer Guide

[rix0rrr]

Since more and more different people work on and contribute to Hedy, expecting new people to absorb architectural decisions from the existing code is not tenable.

This change adds a start for a developer guide with content to guide future implementors.

I'd love suggestions for content to cover here.

[ghost]

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Legend

[Felienne]

Thanks for this @rix0rrr!!

I have moved most of your content to the wiki (see https://github.com/hedyorg/hedy/wiki/Hedy-Developer-Guide).

A few reasons I think this fits best in a wiki:

• Easier to navigate than one long document (newcomers already often don't reach the end of CONTRUBUTING.MD)
• Easier to edit/organize
• Saves PRs/approvals
• Centres the "why" of Hedy (in the main wiki page), encourages people to also read and think about what we are building rather than only how