docs(compiler): Add JSDoc for syntax and source contracts [skip chg] (#12244)

AST consumers need a coherent reference for syntax kinds, source
locations, symbols, declarations, and expressions rather than isolated
undocumented node fields.

Document the public syntax and source contracts together, including
parser limitations, node relationships, optional locations, and
documentation-comment structure. No parser behavior or API signatures
change.

Consolidates [12251](#12251),
[12247](#12247),
[12259](#12259),
[12270](#12270),
[12276](#12276),
[12279](#12279).

Tracked in #12228.

Author: timotheeguerin