feat: add markdown support

[yordis]

No description provided.

[josevalim]

Thank you @yordis! I think the original pull request can be broken into three:

1. Refactor the EPUB and HTML to extract shared logic
2. Effective generation of Markdown pages
3. Add Copy as Markdown button

I think approaching it in steps should help the rebasing process and review. WDYT?

[yordis]

@josevalim it is late for me (3AM), let me work on that then tomorrow.

Refactor the EPUB and HTML to extract shared logic

If you dont mind to share some links to prev comments, or over all your thoughts, otherwise, I will figure it out

[josevalim]

@yordis you can see it in this PR, where epub.ex was accessing HTML directly and now it is extracted into a separate ExDoc.Formatter module :) Ideally, neither Epub nor Markdown modules should access the HTML one.

[yordis]

I tried to merge main in ... I got lost to say the least

[yordis]

@josevalim are you Ok with merging a module that could be unused but unit tested? I am getting lost doing this in chunk to be honest