[doc] Add eMPP and ALP specifications

Test Plan:
Check markdown formatting at
https://github.com/Mengerian/bitcoin-abc/blob/add-alp-spec/doc/standards/alp.md
and
https://github.com/Mengerian/bitcoin-abc/blob/add-alp-spec/doc/standards/empp.md

Reviewers: tobias_ruck, #bitcoin_abc

Reviewed By: tobias_ruck, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D18786