spec issue: limit the elements that can appear inside of a label of a labeled link

[Simon-Laux]

Example:

[Account Sidebar: No window titlebar on MacOS
#3670](https://github.com/deltachat/deltachat-desktop/pull/3670#issuecomment-1935739509)

currently #3670 in the label is parsed as a clickable hashtag. I think it makes sense to limit the allowed elements inside of the label to pure text formatting (+ linebreaks and later emojis for #58).

A clickable hashtag inside of the link label is unexpected for users and in desktop it triggers a react warning/error: Warning: validateDOMNesting(...): <a> cannot appear as a descendant of <a>.

• document the restriction inside of spec.md
• add unit tests for it
• implement the change

[Simon-Laux]

looks like there is also a bug in this where you can send markdown even in the desktop set like this:

[```
hello world
```](https://delta.chat)