add "free text section" at the top

[mybuntu-git2]

Describe your suggested feature

the request is meant to streamline user experience adding a completely custom "intro" section. above the keep reading, a section of editable markdown text with the possibility to insert clickable urls.

useful to have some baseline guidance when something is not found on komga or someone wants to do something. example use cases: " welcome to your webreader! why I can't see my books in komga?? > [url to owncloud books folder] read the manual! > [url to my manual made for very confused people] Dad, remember to add YOUR books to YOUR own library! hey check out this website: [https://libgen.something] "

Other details

I think this should be after the login to prevent exposing stuff. ideally both the title of the section and its content could be configurable

Acknowledgements

• [X] I have searched the existing issues and this is a new ticket, NOT a duplicate or related to another open issue.
• [X] I have written a short but informative title.
• [X] I have updated the app to the latest version.
• [X] I will fill out all of the requested information in this form.