Docs paste plaintext

[godai78]

Closes cksource/ckeditor5-internal#414.

Adding plain text guide with demo. Presents the plain text paste and the special keyboard shortcut plain text paste with style inheritance.

Adding related features section to Paste plain text, Paste from Word, Paste from Google Docs.

Related features placeholder for raw HTML paste.

[AnnaTomanek]

The stable branch cannot reference features that haven't been released.

See the following error that causes the CI to fail:

Error: Failed while convert {@link features/html-embed Raw HTML embedding} tag in ckeditor5/23.0.0/features/paste-from-google-docs.md.
Error: Failed while convert {@link features/html-embed Raw HTML embedding} tag in ckeditor5/23.0.0/features/paste-from-word.md.
Error: Failed while convert {@link features/html-embed Raw HTML embedding} tag in ckeditor5/23.0.0/features/paste-plaintext.md.

[AnnaTomanek]

I cleaned up the text a bit to make it more user-friendly as I don't think the general rumbling about Clipboard was useful to the end user. @Mgsy @FilipTokarski it would be great if you could help with the Installation section code as well as the snippet/build code.

[godai78]

I restored the link removed previously. It would be nice to have this soon, as we want to mention it in the blogpost.

[Mgsy]

I've changed the editor config, reduced a list of plugins used in the build to only formatting-related plugins and added installation instruction for Clipboard plugin.

[godai78]

Looks like this can finally fly.