ci(build_docs): improve build_docs build time (#14912)

## Description

Improve build_docs runtime by enabling `DD_FAST_BUILD=1` and rotating
the cache.

The cache restore/saving time was taking about 8+ minutes.

I updated the cache to be time bound, so it'll rotate once per-month.
After clearing the cache, the build took 10 minutes.

## Testing

<!-- Describe your testing strategy or note what tests are included -->

## Risks

<!-- Note any risks associated with this change, or "None" if no risks
-->

## Additional Notes

<!-- Any other information that would be helpful for reviewers -->

Author: brettlangdon

scripts/docs/build.sh

@@ -2,7 +2,7 @@
 set -eux
 
 # DEV: unless it's built with editable, following sphinx-build fails
-CMAKE_BUILD_PARALLEL_LEVEL=12 CARGO_BUILD_JOBS=12 pip install -v -e .
+DD_FAST_BUILD=1 CMAKE_BUILD_PARALLEL_LEVEL=12 CARGO_BUILD_JOBS=12 pip install -v -e .
 
 if [[ "$(uname)" == "Darwin" ]]; then
   export PYENCHANT_LIBRARY_PATH=/opt/homebrew/lib/libenchant-2.dylib

scripts/gen_gitlab_config.py

@@ -199,6 +199,8 @@ def gen_build_docs() -> None:
             ".readthedocs.yml",
         }
     ):
+        date_str = datetime.datetime.now().strftime("%Y-%m")
+
         with TESTS_GEN.open("a") as f:
             print("build_docs:", file=f)
             print("  extends: .testrunner", file=f)
@@ -212,7 +214,7 @@ def gen_build_docs() -> None:
             print("      hatch run docs:build", file=f)
             print("      mkdir -p /tmp/docs", file=f)
             print("  cache:", file=f)
-            print("    key: v2-build_docs-pip-cache", file=f)
+            print(f"    key: build_docs-pip-cache-{date_str}", file=f)
             print("    paths:", file=f)
             print("      - .cache", file=f)
             print("  artifacts:", file=f)