This PR fixes the links in the concepts section. Most of the links are
not dead links in English version because there are redirects for them.
However, these links will be broken when localized to any languages
other English.
Where appropriate. this PR also wraps some long lines which are
unfriendly to any localization team that try to catch the differences
between revisions.
yanggang
Shubham Kuchhal
Qiming Teng
Sergey Kanzhelev