[ophyd-async] fastcs_connector URI scheme dispatch + "embedding FastCS" docs

## Scope
- `fastcs_connector(uri)` grows scheme dispatch: `pva://` → `PviDeviceConnector` (today), `tango://` → `TangoDeviceConnector`, plus the in-process path taking a `Controller` instance directly (via #399).
- Docs page: "embedding FastCS".

## Acceptance
`uv run --locked tox -e pre-commit,type-checking,tests` green.

ADR: 0019 §4
Parent: #388
Blocked by: #399

Start with ADR 0019 §4, the fastcs_connector entry point, and blocked issue #399 to understand the required dispatch and in-process Controller path. Verify the pva://, tango://, and Controller cases, complete the “embedding FastCS” docs page, then run `uv run --locked tox -e pre-commit,type-checking,tests`; done is a green result.

Written by the indexing model from the issue text.