shift interface in README to docs

[penelopeysm]

human version of #131

also:

• fixed a few typos / broken links
• fixed reference to Setfield.jl (AbstractPPL now uses Accessors)
• add warnings that most of it is outdated and not really used anyway
• add a warning about DensityInterface (it never happened, did it)

[github-actions]

AbstractPPL.jl documentation for PR #144 is available at:
https://TuringLang.github.io/AbstractPPL.jl/previews/PR144/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.28%. Comparing base (575ccfa) to head (56746cf).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #144   +/-   ##
=======================================
  Coverage   86.28%   86.28%           
=======================================
  Files           9        9           
  Lines         452      452           
=======================================
  Hits          390      390           
  Misses         62       62           

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[penelopeysm]

fine copilot you can review if you really want in

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