docs: backport #22515 — Alpha Network page + privacy/limitations updates (v4-next)

[AztecBot]

Backport of #22515 to v4-next.

Summary

• New Alpha Network page at /participate/alpha (docs/docs-participate/alpha.md): known limitations, state-migration expectations across rollup upgrades, security disclosure, path-to-Beta, per-role expectations.
• limitations.md + privacy_considerations.md: document enqueue_incognito for hiding msg_sender, note that hashes/nullifiers/private logs are now padded, drop stale Decoder.sol link, remove duplicate msg_sender section.
• Caution banner in Participate index, sidebar entry, and pointer from networks.md to the new page.

Conflict resolution

Two files conflicted on auto-cherry-pick:

1. docs/CLAUDE.md — trivial timestamp/version block. Took newer values from the PR (2026-04-21 / 1.7).
2. docs/docs/networks.md — terminology mismatch. On next, the mainnet phase is called "Alpha"; on v4-next it's called "Ignition" (Phase 1 — gas limit 0, no user tx, governance/staking active). The PR's replacement paragraph described "transaction processing fully active" which is next-accurate but not v4-next-accurate. Resolution: kept v4-next's Ignition description of Phase 1 specifics, but appended the pointer to the new Alpha Network page so it's still linked from the networks guide.

Full analysis: https://gist.github.com/AztecBot/9076276b1dc2e239f25e3f3983abb53a

Commit structure

Per CLAUDE.md backport protocol:

1. cherry-pick: … — conflicted files staged AS-IS with markers.
2. fix: resolve cherry-pick conflicts — resolves the two markers.

No build-fix commit needed (docs-only).

ClaudeBox log: https://claudebox.work/s/6d396637e42fb57b?run=1