Move Dexter and LB Mi-Cho-Coq proofs in a separate repo

Rationale: save compiling time for Mi-Cho-Coq developers and users.

• dexter 2
• CPMM (LB)

Also needed to update references to the proofs in:

• blog posts
• dexter 2 doc https://gitlab.com/dexter2tz/dexter2tz/-/tree/master
• LB doc https://gitlab.com/dexter2tz/dexter2tz/-/tree/liquidity_baking

This effort is likely to imply some porting effort to adapt the proofs to the 1.0 release of mi-cho-coq.

Note: should probably be done for other big contracts (at least FA1.2 and FA2).