chore(docs): stop readthedocs spellcheck (#15679)

## Description

ReadTheDocs environments don't set `READTHEDOCS=True` as expected.
Update the build script to accept `READTHEDOCS=1` (as set in
`.readthedocs.yml`) and normalize the value (case-insensitive, strip
whitespace) to match both `"true"` and `"1"`.

## Testing

Tested with `"True"`, `"TRUE"`, `"true"`, `"1"`, and `" true "`, all
correctly match. Empty, `"False"`, and `"0"` correctly do not match.

## Additional Notes

Failing job:
https://app.readthedocs.org/projects/ddtrace/builds/30721096/

Author: mabdinur

.readthedocs.yml

@@ -8,5 +8,5 @@ build:
     - cargo install --force --root /home/docs/.asdf --git https://github.com/DataDog/libdatadog --bin dedup_headers tools
     - git fetch --unshallow || true
     - pip install riot
-    - riot -v run --pass-env build_docs
+    - READTHEDOCS=1 riot -v run --pass-env build_docs
     - mv docs/_build $READTHEDOCS_OUTPUT

scripts/docs/build.sh

@@ -1,7 +1,9 @@
 #!/usr/bin/env bash
 set -eux
 
-if [[ "${READTHEDOCS:-}" = "True" ]]; then
+# Normalize READTHEDOCS: lowercase and strip whitespace to match "true" or "1"
+rtd_val="${READTHEDOCS:-}"; rtd_val="${rtd_val,,}"; rtd_val="${rtd_val// /}"
+if [[ "$rtd_val" == "true" || "$rtd_val" == "1" ]]; then
   echo "Skipping spelling check in RTD"
 else
   if [[ "$(uname)" == "Darwin" ]]; then