chore(gitignore): add docs/build/ to ignored paths

## Starting point

The project's Sphinx configuration outputs to docs/build/, but only
docs/_build/ was listed in .gitignore. The actual build directory was
untracked by coincidence rather than by rule.

## Changes

Added docs/build/ entry alongside the existing docs/_build/ entry in the
documentation builds section of .gitignore. Both entries are kept because
docs/_build/ is the Sphinx default and docs/build/ is the path configured
in this project's Makefile/conf.

## Result

Sphinx build artifacts in docs/build/ are now explicitly ignored,
preventing accidental commits of generated HTML files.

Author: FirstUnicorn

.gitignore

@@ -63,6 +63,7 @@ dmypy.json
 
 # Documentation builds / Sphinx docs
 docs/_build/
+docs/build/
 docs/.doctrees/
 docs/source/autoapi/
 docs/autoapi/