Docs: Update CLAUDE.md and rbac.md for accuracy

## Description

Multiple documentation inconsistencies found:

1. **CLAUDE.md** - Missing `MCPServerRegistration` from MCP resources list
2. **CLAUDE.md** - `MCPOverviewPage` description is outdated
3. **docs/rbac.md** - Contradictory all-namespaces behavior description

## Current Behaviour

**CLAUDE.md:65-68** - MCP Resources section lists:
- ✅ `MCPGatewayExtension`
- ✅ `ReferenceGrant`
- ❌ Missing: `MCPServerRegistration` (actually used in MCPOverviewPage)

**CLAUDE.md:145-148** - Says:
> "Empty state entry point for MCP Management with link to setup wizard"

But MCPOverviewPage now renders:
- Health statistics
- Resource tables (extensions, servers, grants)
- Attached policies
- RBAC-gated panels
- Empty state is just one conditional branch

**docs/rbac.md:7** - Says:
> "all-namespaces checks are cluster-scoped"

This contradicts actual `#ALL_NS#` implementation.

## Expected Behaviour

**Fix 1:** Add to CLAUDE.md MCP Resources:
```markdown
14. **MCPServerRegistration** (`mcp.kuadrant.io/v1`) - MCP server configuration and registration
```

**Fix 2:** Update CLAUDE.md MCPOverviewPage description:
```markdown
**MCPOverviewPage**: MCP management dashboard with gateway/server health statistics, resource tables (MCPGatewayExtension, MCPServerRegistration, ReferenceGrant), attached policies list, and RBAC-gated panels. Shows setup wizard empty state when no extensions exist.
```

**Fix 3:** Audit and fix `docs/rbac.md` all-namespaces behavior to match implementation.

## Locations

- `CLAUDE.md:65-68` (MCP resources)
- `CLAUDE.md:145-148` (MCPOverviewPage)
- `docs/rbac.md:7` (all-namespaces)

## Related

CodeRabbit comment in PR #734

Start with CLAUDE.md:65-68 and 145-148, then inspect docs/rbac.md:7 and the implementation of the #ALL_NS# behavior. Update the MCP resource list and MCPOverviewPage description, and make the all-namespaces documentation match the implementation; verify all three locations for consistency.

Written by the indexing model from the issue text.