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

[coretl]

Scope

• fastcs_connector(uri) grows scheme dispatch: pva:// → PviDeviceConnector (today), tango:// → TangoDeviceConnector, plus the in-process path taking a Controller instance directly (via [ophyd-async] Embedded FastCS connector (fastcs-embed extra) #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