Add support for multipage documents

[Ndpnt]

Opened to allow pre-review. I still have to:

• update the history schema for validation command
• maybe some factorization in archivist/index.js

[Ndpnt]

To test, you can use convert-declarations-to-multipages branch on france-elections-declarations repository.

[martinratinaud]

Branch experiment-multipages rebased on this one has been deployed on the experiment server.

It takes into account declarations from convert-declarations-to-multipages

Example of combined pages can be found here https://github.com/OpenTermsArchive/sandbox-versions/blob/main/YouTube/Community%20Guidelines.md