feat: migrate from edoc to ex_doc

[puzza007]

Summary

• Replace edoc with ex_doc (via rebar3_ex_doc) for modern, searchable hex docs
• Add OTP 27 -moduledoc and -doc attributes to all modules
• Replace %% @private / %% @equiv comments with -doc false / -doc #{equiv => ...}
• Hide internal modules (katipo_app, katipo_sup, katipo_metrics) from generated docs
• Include README and LICENSE as extra pages

Test plan

• rebar3 compile passes
• rebar3 ex_doc generates docs successfully
• CI passes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.78%. Comparing base (29e9642) to head (6529e77).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #171   +/-   ##
=======================================
  Coverage   72.78%   72.78%           
=======================================
  Files           6        6           
  Lines        1062     1062           
=======================================
  Hits          773      773           
  Misses        289      289           

Flag	Coverage Δ
c	72.78% <ø> (ø)
erlang	72.78% <ø> (ø)

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