Added RTSP documentation to Amcrest camera

[tchellomello]

Description:

Added RTSP documentation to Amcrest camera.

#cameras.yaml
- platform: amcrest
  name: "Amcrest Backyard"
  host: !secret amcrest_backyard_host
  username: !secret amcrest_backyard_username
  password: !secret amcrest_backyard_password
  port: 80
  resolution: low
  stream_source: rtsp

- platform: amcrest
  name: "Amcrest Driveway"
  host: !secret amcrest_driveway_host
  username: !secret amcrest_driveway_username
  password: !secret amcrest_driveway_password
  port: 80
  resolution: high
  stream_source: rtsp

Pull request in home-assistant (if applicable): home-assistant/core#7646

[mention-bot]

@tchellomello, thanks for your PR! By analyzing the history of the files in this pull request, we identified @fabaff and @colinodell to be potential reviewers.

[Landrash]

Looks good and can be merged after parent PR is merged.