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 dev_4_4 -D org -I include -E conflict -E needs-fixes -E broken: PR 245 (Add note about Java UTF-8 source in Insight.)
merge dev_4_4 -D org -I include -E conflict -E needs-fixes -E broken: PR 266 (Add section about the Sphinx linkcheck output parsing)