docs(mcp): document multi-index server contract (RAAE-1608)

[vishal-bala]

Motivation

The RedisVL MCP docs were written for the original single-index model and still stated that "one server process binds to exactly one existing Redis index." With multi-index support now implemented across RAAE-1604–RAAE-1607, the user-facing documentation (RAAE-1608) needs to describe the new compatibility story without confusing existing single-index users.

The guiding principle in the rewrite is that single-index remains the simplest deployment and works exactly as before — callers never name an index — while multi-index is presented as a formal, additive capability layered on top via discovery (list-indexes) and explicit routing (the index argument). No documentation still claims a server must bind to exactly one index.

Implemented changes

The concept doc (concepts/mcp.md) now frames the server as binding one or several logical indexes, each addressed by an id, and adds an "Index Selection and Discovery" section covering the optional index argument, the omitted-index and unknown-id rules, the response echo, and discovery-first guidance. The "Single Index Binding" section becomes "Single and Multiple Index Bindings," the read-only section explains the two-level write policy (global --read-only vs per-index read_only, folded into effective write availability), and the tool surface gains a list-indexes subsection documenting its minimal payload — filterable fields only, vector/embed-source fields omitted, explicit-only limits, and redis_name never exposed.

The how-to guide (how_to_guides/mcp.md) adds a two-binding config example (a writable vector index alongside a read-only fulltext index), an Index Selection subsection, a list-indexes tool contract with a response example, and threads the optional index argument through the search-records and upsert-records argument lists and request/response examples. A discovery-first multi-index flow is shown at the top of the search examples, and the CLI/env-var notes are updated for per-index read-only and the multi-index search description.

Minor additional changes:

• README MCP section and feature-table entry reworded from "an existing Redis index" to "one or more existing Redis indexes," with list-indexes and the discovery-first flow noted. (The README edit was explicitly authorized for this ticket, overriding the repo's default no-README-edits rule.)

Verification

• sphinx-build (the docs dependency group) completes cleanly (exit 0). The only warnings are pre-existing and unrelated (upstream redis-py docstrings and index.md heading levels); no warnings reference the MCP pages, and no broken cross-reference/anchor warnings were introduced.

Stacking

This PR targets feature/raae-1607-upsert-routing and is the top of the stack. Review/merge bottom-up: #629 → #630 → #631 → #632 → this PR.

🤖 Generated with Claude Code

[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

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