Align docs structure + drop version clutter #228

[rymsha]

Closes #228 (cherry-pick of the master-branch cleanup — PR #229 — onto 4.x).

Same doc changes as #229. On this branch the change to .github/workflows/enonic-docgen.yml is smaller because 4.x was already listed there; this PR reorders the entry to sit next to master for consistency with the copy on master.

The 3.x (XP 7) branch is deliberately left untouched.

Structure

• docs/index.adoc: add NOTE: Versions 4.x target XP 8+. near the top.
• docs/versions.json: 4.x becomes stable (latest: true) with 3.x as the second entry (drops the pre-release next / master pairing).
• .github/workflows/enonic-docgen.yml: keep master, 4.x, 3.x — every branch listed in versions.json plus master.

Clutter removed from docs/index.adoc

• image::…badge/xp-7.+… — XP 7+ shield no longer reflects the target platform (4.x is XP 8+).
• include 'com.enonic.lib:lib-http-client:3.2.1' → include "com.enonic.lib:lib-http-client:${libVersion}". The hardcoded 3.2.1 was frozen from an earlier version.
• (added in v3.2.0) on the disableHttp2 option.
• [*v3.0.0+*] on the KeyStore and TrustStore Configuration section heading.
• Three of four bullets in == Compatibility:
  • "Not compatible with XP releases before version 7.0 …" — pre-XP 8 caveat that no longer applies.
  • "Starting from version 3.0.0 library uses Java HttpClient instead of OkHttp …" — describes the 2.x → 3.x transition.
  • "Starting from version 3.0.0 library uses Java Platform KeyStore …" — duplicates the KeyStore and TrustStore Configuration section above and describes a removal readers no longer need to know about.

Deliberately kept

• The Preemptive authentication is not supported. Use an Authorization: Basic <credentials> header instead. bullet — reworded to current tense.
• KeyStore and TrustStore Configuration section body (including the WARNING) — current behavior.
• Compression section — current behavior.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.42%. Comparing base (815c9cb) to head (6460ff1).

Additional details and impacted files

@@            Coverage Diff            @@
##                4.x     #230   +/-   ##
=========================================
  Coverage     91.42%   91.42%           
  Complexity      156      156           
=========================================
  Files             9        9           
  Lines           513      513           
  Branches         58       58           
=========================================
  Hits            469      469           
  Misses           23       23           
  Partials         21       21           

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