renamed build_docs.sh to build_antora.sh as per ci_boost_release.py requisites

[joaquintides]

Antora currently has a limitation by which it must be run in a proper Git repo (as opposed to a submodule of a superproject). The Boost release script fixes that by doing some Git adjustments on those submodules it identifies as bulding Antora docs, which identification is done heuristically by looking for an antora_docs.sh or build_antora.sh file:

https://github.com/boostorg/release-tools/blob/d27012edbe2516ce8c38798244a153a216ccba95/ci_boost_release.py#L495-L536

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.56%. Comparing base (354c63c) to head (1235291).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop      #98   +/-   ##
========================================
  Coverage    96.56%   96.56%           
========================================
  Files            5        5           
  Lines          785      785           
  Branches       233      233           
========================================
  Hits           758      758           
  Misses          27       27           

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