Add compile-requirements GH workflow and update requirements README (#33126)

Adds a GH workflow that will run `make compile-requirements` and create
a PR for it and overhauls the requirements README with info on how to
use the new workflows.

README changes:

- Group the workflow/makefile info under a new heading
- Switch to imperative for headings
- Move "Upgrade all dependencies" to end of list (uncommon for people to
  actually need this command, I suspect)
- Add "Add a dependency"
- Preamble about .in and .txt files and Mac vs. Linux, as well as about forks

Also, some style fixes to upgrade-one-python-dependency GH workflow:

- Fix indentation in inputs block
- Remove "-workflow" from job name (probably copied from another workflow)
- Wrap a long line

.github/workflows/compile-python-requirements.yml

@@ -0,0 +1,72 @@
+name: Recompile Python dependencies
+
+on:
+  workflow_dispatch:
+    inputs:
+      branch:
+        description: 'Target branch to create requirements PR against'
+        required: true
+        default: 'master'
+        type: string
+
+defaults:
+  run:
+    shell: bash # making this explicit opts into -e -o pipefail
+
+jobs:
+  recompile-python-dependencies:
+    runs-on: ubuntu-20.04
+
+    steps:
+      - name: Check out target branch
+        uses: actions/checkout@v3
+        with:
+          ref: "${{ inputs.branch }}"
+
+      - name: Set up Python environment
+        uses: actions/setup-python@v4
+        with:
+          python-version: "3.8"
+
+      - name: Run make compile-requirements
+        env:
+          PACKAGE: "${{ inputs.package }}"
+        run: |
+          make compile-requirements
+
+      - name: PR preflight
+        run: |
+          if git diff --exit-code; then
+              # Fail early (and avoid quiet failure of create-pull-request action)
+              echo "Error: No changes, so not creating PR." | tee -a "$GITHUB_STEP_SUMMARY"
+              exit 1
+          fi
+
+      - name: Make a PR
+        id: make-pr
+        uses: peter-evans/create-pull-request@v5
+        with:
+          branch: "${{ github.triggering_actor }}/compile-python-deps"
+          branch-suffix: short-commit-hash
+          add-paths: requirements
+          commit-message: |
+            feat: Recompile Python dependencies
+
+            Commit generated by workflow `${{ github.workflow_ref }}`
+          title: "chore: Recompile Python dependencies"
+          body: >-
+            PR generated by workflow `${{ github.workflow_ref }}`
+            on behalf of @${{ github.triggering_actor }}.
+          assignees: "${{ github.triggering_actor }}"
+          reviewers: "${{ github.triggering_actor }}"
+
+      - name: Job summary
+        env:
+          PR_URL: "${{ steps.make-pr.outputs.pull-request-url }}"
+        run: |
+          if [[ -z "$PR_URL" ]]; then
+              echo "PR not created; see log for more information" | tee -a "$GITHUB_STEP_SUMMARY"
+              exit 1
+          else
+              echo "PR created or updated: $PR_URL" | tee -a "$GITHUB_STEP_SUMMARY"
+          fi