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/upgrade-one-python-dependency.yml

@@ -2,32 +2,32 @@ name: Upgrade one Python dependency
 
 on:
   workflow_dispatch:
-     inputs:
-       branch:
-         description: 'Target branch to create requirements PR against'
-         required: true
-         default: 'master'
-         type: string
-       package:
-         description: 'Name of package to upgrade'
-         required: true
-         type: string
-       version:
-          description: 'Version number to upgrade to in constraints.txt (only needed if pinned)'
-          default: ''
-          type: string
-       change_desc:
-         description: |
-           Description of change, for commit message and PR. (What does the new version add or fix?)
-         default: ''
-         type: string
+    inputs:
+      branch:
+        description: 'Target branch to create requirements PR against'
+        required: true
+        default: 'master'
+        type: string
+      package:
+        description: 'Name of package to upgrade'
+        required: true
+        type: string
+      version:
+        description: 'Version number to upgrade to in constraints.txt (only needed if pinned)'
+        default: ''
+        type: string
+      change_desc:
+        description: |
+          Description of change, for commit message and PR. (What does the new version add or fix?)
+        default: ''
+        type: string
 
 defaults:
   run:
     shell: bash # making this explicit opts into -e -o pipefail
 
 jobs:
-  upgrade-one-python-dependency-workflow:
+  upgrade-one-python-dependency:
     runs-on: ubuntu-20.04
 
     steps:
@@ -92,8 +92,9 @@ jobs:
 
             ${{ env.body_prefix }}Commit generated by workflow `${{ github.workflow_ref }}`
           title: "feat: Upgrade Python dependency ${{ inputs.package }}"
-          body: |
-            ${{ env.body_prefix }}PR generated by workflow `${{ github.workflow_ref }}` on behalf of @${{ github.triggering_actor }}.
+          body: >-
+            ${{ env.body_prefix }}PR generated by workflow `${{ github.workflow_ref }}`
+            on behalf of @${{ github.triggering_actor }}.
           assignees: "${{ github.triggering_actor }}"
           reviewers: "${{ github.triggering_actor }}"