Add section about the Sphinx linkcheck output parsing

- Add warning priority table based on the link type/error code
- Add warning thresholds table marking the build as unstable
Add fixes suggested by Helen and improve threshold table
merge develop -D org -I include -E conflict -E needs-fixes -E broken: PR 268 (User docs develop)
merge develop -D org -I include -E conflict -E needs-fixes -E broken: PR 269 (Add section about the Sphinx linkcheck output parsing (rebased onto develop))