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

[OniriCorpe]

Reverts #2428

Original PR: #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

• 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
• PR finished and ready to be reviewed

[OniriCorpe]

uuuuh omg i readed the whole docs 3 times lmao 🙃😓😭

ready to review 🎉