docs: reference-manual page for fine-grained module definitions

Documents `module M' = M with {...}`: syntax, code positions, the
copy/share semantics, restrictions, and worked examples (ghost-state
instrumentation, statement and result rewrites, loop edits,
match-branch collapse, functor and sub-module updates).

The section on sub-module and applied-functor bases documents the
semantics introduced by the parent commit (sound module updates on
sub-module and applied-functor bases); its example is executable and
mirrors the regression tests in tests/module-update-bases.ec.  This
change is meant to be merged after that fix.

The page is marked :orphan: until a language-reference toctree lands.

Author: Gustavo2622