Link: Detect dead link

[benjaminParisel]

IMPORTANT: #594 propose to add some tooling to solve the problems listed here

With Antora 3, we can detect if Xref link is valid or not.
But in our content, some external links exist. We need to be sure all link is valid and exist to avoid a bad impact on SEO.
This will also validate that we don't have locahost links, or texts generated as links.
Notice taht this is slower that the xref validation

To do this, some tools exist:

• https://github.com/celinekurpershoek/link-checker
• https://github.com/marketplace/actions/broken-link-checker-action
• https://linkchecker.github.io/linkchecker/man/linkchecker.html#synopsis
• tips from Antora zulip instance: https://antora.zulipchat.com/#narrow/stream/282400-users/topic/Link.20checking
  https://github.com/wjdp/htmltest
• https://www.drlinkcheck.com/account/subscriptions/1/projects/1/reports/blocked
• https://validator.w3.org/: tested by Adrien Lachambre in November 2021. Almost no broken links detected
• https://github.com/JustinBeckwith/linkinator

Already known broken links (non exhaustive list)

• link http://documentation.bonitasoft.com/ inside xsd files or ref
• pages about the comparison between Bonita Forms v6 and v7 for anonymous user

Linkchecker past run

2021-12-08, run linkchecker on production site for bonita version 2021.1 (or 2021.2). It tooks 5 to 15 minutes to complete.
It detected issues caused by the Markdown to AsciiDoc migration. localhost:<port> converted into automatic HTML links. They should be escaped with + symbols.

linkchecker -o html https://documentation.bonitasoft.com/bonita/2021.2/ | tee ~/_tmp/linkchecker_bonita_2021.2/bonita_2021.2_new.html