Move HACKING.rst to readthedocs

[mr-cal]

1. Migrate HACKING.rst to starbase documentation such that applications can re-use it in their documentation
2. Softlink or add a note in HACKING.rst pointing to the documentation
   • This is because developers except to see a hacking guide in the root of a repo

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The internal ticket has been created: https://warthogs.atlassian.net/browse/CRAFT-3173.

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[medubelko]

Softlink or add a note in HACKING.rst pointing to the documentation

For metadocs about the codebase, I'd rather we keep files in a format where they're mostly readable without rendering/building or a web browser.

On that note, I'd like to also move away from HACKING as a prevailing file, as the title has negative connotations and the file isn't much of a common practice. Many projects one runs across on GH carry get-up-and-go build instructions early in the README, or sometimes none at all if it's as simple as make install. This means that there will be less DRY between repos, yes, but there's a limit to how much we can modularise historically non-Sphinx documents meant to be read in source. Besides, if we tidy up and consolidate the build, it should be a relatively short section (I hope). If it's on the long side due to testing or whatnot, we can explore other solutions.