Lint check: links and section header labels

- [ ] add check for anchors uses in links that go to our own docs (consider how this would change if we switched to using the Sphinx interdocs links plugin)

- [ ] add check for section headings without an explicit ID. to avoid heading rewordings from breaking existing links (e.g., https://crate.io/docs/crate/reference/en/latest/general/builtins/table-functions.html#generate-series-start-stop-step)