docs: env var doc improvements (#8777)
## Summary
* Env docs now support anchors, which allows sending a link to someone
with a direct reference to an env var or cross-reference them in the
docs.
* Marked additional env vars as hidden from the docs due to their
internal use
* Updates some tests still using literals to use the static env vars
## Test Plan
<img width="1370" alt="env_var_anchors"
src="https://github.com/user-attachments/assets/52ae1caa-5199-4798-9eb5-81b8f5b57c24">
