WIP: Add little more docs about plugins

[selfhoster1312]

I added a little more documentation about what i found confusing when making my first plugin, adding a page about custom forms.

I also added information about https://github.com/kanboard/kanboard/issues/5695.

Let me know if i should change some things. Feel free to edit and push directly to my branch. I could make french translation if/when PR is accepted.

[selfhoster1312]

I think the syntax highlighter is not very happy with the <?= syntax:

I'm not sure what to do about this.

[fguillot]

try to use phtml instead of php in the Markdown block

```phtml
[...]