add support doc for spamassassin module

[juliebin]

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Note

Low Risk
Low risk: documentation-only changes (new page + TOC/changelog entries) with no runtime or configuration behavior changes.

Overview
Adds a new reference page for the supported spamassassin (SPAMC) client module, including configuration options, protocol expectations, context variables, and Lua/C usage examples.

Updates the modules index and the all-modules summary to include spamassassin, and adds a Momentum 5.3.0 changelog entry noting it replaces the legacy Sieve-based spamc module.

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