Improve markdown formatting of pages, a lot of them are improperly formatted

[OniriCorpe]

Reverts YunoHost/doc#2428

Original PR: https://github.com/YunoHost/doc/pull/2427

Problem

• many pages are really improperly formatted 😅😅😅😅

Solution

• i ran my markdown formater over all the pages and manually corrected a lot of things I could see
• it'll deserve a second in-depth manual pass to replace the irrelevant in-line HTML
• add .markdownlint.json to standardize the doc's markdown
• add https://github.com/syseleven/markdownlint-rules-grav-pages to add grav special things to the automatic markdown lint, but it's just cloned in the repo, if someone know a better way of doing it, don't hesitate, plz <3

TODO

• [ ] github action ou tests to lint PR

PR checklist

• [x] I'm not doing a PR for an application, I promise, I know that this kind of changes must go directly into the app packages themselves
• [x] PR finished and ready to be reviewed