* doc: improve fetchers overview, deduplicate readme content
* Improve caveat explanation and some fetchurl content
* move out consumer docs on source fetching
* move note on mirror URLs to the relevant section
this may be better suited for the `fetchurl` reference, but it's probably better to
just render that information into the manual. for now, because
- contributor documentation encourages mirrors
- we can expect contributors to dig into the source
- linking source files is trivial in in-code documentation
we leave it there.
* move instructions for updating hashes to the manual
* Add more clarity on text, reorganise source hash methods
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Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>
Co-authored-by: Dominic Mills-Howell <dominic.millz27@gmail.com>
Co-authored-by: lolbinarycat <dogedoge61+github@gmail.com>
