add support doc for spamassassin module

[juliebin]

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Note

Low Risk
Documentation-only change that adds a new module page and links it into the module indexes; low risk aside from potential doc inaccuracies or broken links.

Overview
Adds a new spamassassin module reference page describing configuration (daemon, timeout, max_size), SPAMC/SPAMD protocol expectations, exposed spamc_* message context variables, and Lua/C entry points for invoking scans.

Updates the modules TOC (modules/index.md) and the full module summary table (summary-all-modules.md) to include the new spamassassin module entry and note it as a singleton introduced in 5.0 that replaces the legacy Sieve-based spamc module.

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