ci: set required memory request/limit for build_docs job (#13363)

With VPA now disabled, the default memory request/limit of 2Gi is not
enough for this job.

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [x] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
- Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

Author: brettlangdon

scripts/gen_gitlab_config.py

@@ -120,11 +120,19 @@ def gen_build_docs() -> None:
             print("  extends: .testrunner", file=f)
             print("  stage: hatch", file=f)
             print("  needs: []", file=f)
+            print("  variables:", file=f)
+            print("    PIP_CACHE_DIR: '${CI_PROJECT_DIR}/.cache/pip'", file=f)
+            print("    KUBERNETES_MEMORY_REQUEST: 4Gi", file=f)
+            print("    KUBERNETES_MEMORY_LIMIT: 4Gi", file=f)
             print("  script:", file=f)
             print("    - |", file=f)
             print("      hatch run docs:build", file=f)
             print("      mkdir -p /tmp/docs", file=f)
             print("      cp -r docs/_build/html/* /tmp/docs", file=f)
+            print("  cache:", file=f)
+            print("    key: v0-2.21-build_docs-pip-cache", file=f)
+            print("    paths:", file=f)
+            print("      - .cache", file=f)
             print("  artifacts:", file=f)
             print("    paths:", file=f)
             print("      - '/tmp/docs'", file=f)