Ckeditor5: Add a FAQ section in the docs

[Reinmar]

Questions:

• [ ] Why the editor filters out my content (styles, classes, attributes, elements)? And why there's no allowContent: true?
• [ ] This place is waiting for you

[scofalik]

• How to turn on source mode / How to write a source mode plugin

[Reinmar]

Great ideas, thanks! What else? :)

[Reinmar]

• Is there a Classic editor full package build with a full toolbar? (source https://github.com/ckeditor/ckeditor5/issues/860)

[Reinmar]

How to replace CKEditor 5 icons? (Answer: by using NormalModuleReplacementPlugin)

[scottmessinger]

I've been looking through the docs and I'm a bit confused as to the format the data is exported in. In the docs, getData is referenced, but it's unclear what format the string is -- is it using the XML processor or the HTML one? Also, if we wanted to go all-in on ckeditor and save text in our database in the most optimal, highest fidelity, fastest-for-ckeditor-to-parse format, how would I do that? Is there a JSON export that would work best or is the XML data format the best?

[Reinmar]

Would you mind to ask this question on StackOverflow? It'd be great if we had an answer there.

[Reinmar]

PS. In the meantime, you can also check https://github.com/ckeditor/ckeditor5/issues/620.