InfluxDB 3 Cloud: add OpenAPI spec for the initial (PR #7338) endpoint set

## Summary

Add an OpenAPI spec (and generated API reference) for InfluxDB 3 Cloud, scoped to **only the endpoints that back the features included in #7338**:

- **Write**: `/api/v3/write_lp` (and v1/v2 write-compatibility endpoints if in scope)
- **Query**: `/api/v3/query_sql`, `/api/v3/query_influxql`
- **Databases**: `/api/v3/configure/database` (create, list, delete)
- **Tables**: `/api/v3/configure/table` (create, delete)
- **Tokens**: admin token and database (resource) token endpoints under `/api/v3/configure/token`

## Why

The Cloud CLI reference added in #7511 is deliberately restricted to the #7338 feature set (write, query, databases, tables, named admin tokens, database tokens). The API reference should match that surface exactly — no Processing Engine, caches, file index, system, or operator endpoints — so CLI and API docs stay consistent for the initial release.

## Scope / notes

- Exclude endpoints for features not in this release (Processing Engine/plugins/triggers, caches, file index, nodes/license, etc.).
- Mirror the CLI split: reuse the Enterprise/Core spec where identical; keep Cloud-specific (hosted) differences separate.
- Related: #7338 (initial Cloud docs), #7511 (Cloud CLI reference).

Start by comparing the API surface described here with the initial Cloud feature set in #7338 and the CLI reference in #7511. Identify the existing Enterprise/Core and Cloud OpenAPI specs, then document the listed write, query, database, table, and token endpoints; done means the generated API reference matches that set and excludes the explicitly out-of-scope features.

Written by the indexing model from the issue text.