FINERACT-2171: Make Slack messages discoverable by search engines via AsciiDoc archiving

[airajena]

Description:

This PR implements an automated mechanism to fetch public Slack conversations and archive them as static AsciiDoc files within the Fineract documentation. This directly addresses FINERACT-2171 by making ephemeral community knowledge indexable by search engines and permanently accessible.

Changes

• New Script: Added fineract-doc/slack.gradle to handle the fetching and generation logic.
• Build Integration: Updated fineract-doc/build.gradle to include the archiving task as a dependency of asciidoctor.
• Documentation: Updated the main index.adoc to include the new "Slack Archive" chapter.
• Safeguards: Implemented robust checks for missing tokens (CI/CD safe), rate limiting, and privacy control using an allowlist mechanism.

Testing

• Local Build: Verified that ./gradlew :fineract-doc:asciidoctor succeeds both with and without SLACK_TOKEN.
• Formatting: Verified that generated AsciiDoc tables render correctly with timestamps and usernames.
• Code Quality: Ran spotlessApply to ensure the new Gradle script follows project formatting standards.

Related Issue

Fixes FINERACT-2171