fix(docs): Improve accuracy and fix broken links in L1-L2 messaging documentation

[critesjosh]

Summary

Reviewed and updated the L1-L2 messaging (Portals) documentation for accuracy against the current source code implementation.

Changes to index.md (Portal overview)

• Fixed incorrect constant MAX_MESSAGESTACK_DEPTH → MAX_L2_TO_L1_MSGS_PER_TX
• Fixed inaccurate kernel circuit description (clarified that kernel scopes messages to emitting contracts, and accumulates nullifiers rather than creating them)
• Removed outdated "contracts tree" terminology
• Fixed dangling "previous section" reference
• Fixed grammar and improved clarity throughout
• Added "See also" section with links to related pages
• Removed non-standard importance field from front-matter

Changes to inbox.md

• Fixed broken computeSecretHash link (was pointing to non-existent yarn-project/aztec.js/src/utils/secrets.ts, now correctly points to yarn-project/stdlib/src/hash/hash.ts)

Changes to registry.md

• Changed hardcoded master branch links to use #include_aztec_version macro
• Improved introductory description
• Added documentation for missing view functions (getVersion(), getGovernance(), getRewardDistributor())
• Added "Related pages" section

Verified as accurate (no changes needed)

• data_structures.md - All #include_code tags verified
• outbox.md - Edge cases and error messages verified against implementation

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