internal/readme: use go:generate rather than Make

Simplify the readme generation to just use go:generate, no longer
depending on Make or bash.

Subsequent CLs will generate more documentation in a similar manner.

For #442

.github/workflows/readme-check.yml

@@ -19,14 +19,13 @@ jobs:
       uses: actions/checkout@v4
     - name: Check README is up-to-date
       run: |
-        cd internal/readme
-        make
+        go generate ./internal/readme
         if [ -n "$(git status --porcelain)" ]; then
           echo "ERROR: README.md is not up-to-date!"
           echo ""
-          echo "The README.md file differs from what would be generated by running 'make' in internal/readme/."
+          echo "The README.md file differs from what would be generated by `go generate ./internal/readme`."
           echo "Please update internal/readme/README.src.md instead of README.md directly,"
-          echo "then run 'make' in the internal/readme/ directory to regenerate README.md."
+          echo "then run `go generate ./internal/readme` to regenerate README.md."
           echo ""
           echo "Changes:"
           git status --porcelain