chore: fix docs builds [backport 3.19] (#15540)

Backport fdd8fed from #15533 to 3.19.

## Description

Our docs are failing to build in readthedocs but the build_docs job is
passing in gitlab
([passing](https://gitlab.ddbuild.io/DataDog/apm-reliability/dd-trace-py/-/jobs/1269199858/viewer),
[failing](https://app.readthedocs.org/projects/ddtrace/builds/30557822/)).

Fixed ReadTheDocs build failures by skipping spelling checks in
ReadTheDocs. Different spelling backends between GitLab CI and
ReadTheDocs cause inconsistent results (e.g., "jitter", "durations" pass
in CI but fail in RTD).


Changes:
- Skip spelling check in ReadTheDocs (validated in CI)

## Testing

- Verified spelling check passes locally
- GitLab CI continues to run all validation checks

## Risks

None

Co-authored-by: Munir Abdinur <[email protected]>

Author: dd-octo-sts[bot]

docs/spelling_wordlist.txt

@@ -346,3 +346,5 @@ xfail
 yaaredis
 openai-agents
 validators
+jitter
+durations

scripts/docs/build.sh

@@ -2,18 +2,13 @@
 set -eux
 
 if [[ "${READTHEDOCS:-}" = "True" ]]; then
-    # We skip here because we do not check spelling in RTD
-    echo "Skipping install"
+  echo "Skipping spelling check in RTD"
 else
   if [[ "$(uname)" == "Darwin" ]]; then
     brew install enchant
+    export PYENCHANT_LIBRARY_PATH=$(brew --prefix enchant)/lib/libenchant-2.dylib
   fi
+  sphinx-build -vvv -W -b spelling docs docs/_build/spelling
 fi
-
-if [[ "$(uname)" == "Darwin" ]]; then
-  export PYENCHANT_LIBRARY_PATH=/opt/homebrew/lib/libenchant-2.dylib
-fi
-
 reno lint
-sphinx-build -vvv -W -b spelling docs docs/_build/html
 sphinx-build -vvv -W -b html docs docs/_build/html