[doc] Initial getblocktemplatelight specification

Draft of getblocktemplatelight / submitblocklight RPC API, for review & discussion.

Test plan:

• review

Pre-merge:

• bump version number to 1.0, take it out of draft done in latest commit

Merge instructions:

• squash it :) (please use "[doc] Initial getblocktemplatelight specification" as squash commit title)