help: Also mention Pitivi's license on the user manual's license page
help: Add a tip on how to remove middle portions of clips in splitting.page
help: Update importing.page, movearoundtimeline.page and usingclips.page
help: Create a new page in the user manual to explain grouping
help: Add the "keyseq" tag where appropriate

- Use it instead of <key>key</key>+<key>key</key> in movearoundtimeline.page
- Use it instead of <key>key+key</key> in cheatsheet.page
help: Document better the procedure of creating new layers

Fixes bug #720878