docs: add architecture overview and participant architecture

[cgdusek]

Summary

• Add an Architecture section to README.md with a Mermaid diagram showing how participant nodes, sequencers, and mediators interact within a synchronizer
• Flesh out the previously empty participant-architecture.rst in docs-open/ with component descriptions, command processing flow, multi-synchronizer and HA cross-references

Closes #390

Context

PR #391 previously attempted to address this issue but was closed because:

1. Docs were placed in docs/architecture/ instead of docs-open/
2. The diagram contained architectural inaccuracies (topology manager, direct participant links, wrong mediator flow)
3. At the time, docs lived in a separate repository

Per @soren-da's guidance, docs now live in docs-open/ and contributions are welcome there.

Changes

README.md — New Architecture section with:

• Mermaid diagram matching the canonical example from protocol.rst (Alice multi-hosted on P1/P3, Bob on P1, Charlie on P2, two sequencers, one mediator)
• Key architectural invariants (no direct participant communication, total-order multicast, two-phase commit via mediator)
• Links to in-repo docs and published documentation

participant-architecture.rst — Replaced empty WIP stub with:

• Overview of all major participant components (Ledger API, Admin API, Daml Engine, Synchronizer Router, Transaction Processor, Sequencer Client, Contract Store, Indexer)
• 7-step command processing flow from submission to commit
• Multi-synchronizer support and HA sections with cross-references to existing pages

Architectural accuracy

All content verified against protocol.rst, participant.rst, synchronizers.rst, and the maintainer review comments on #391:

• No Topology Manager (removed in 3.x)
• No direct participant-to-participant links
• Mediator connects to sequencers (bidirectional), not to participants
• Matches the Alice/Bob/Charlie/P1/P2/P3 example from protocol.rst

Test plan

• Verify Mermaid diagram renders on GitHub (visible in PR diff or README preview)
• Confirm RST cross-references resolve (synchronizer-architecture, protocol-sequencer-nodes, reassignment-protocol, multiple-synchronizers, participant-ha)
• Review architectural accuracy against published docs

[github-actions]

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[cgdusek]

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