[PXCT-1779] document custom bricks

[swikstrom]

What This PR Changes

• Document custom bricks

Contribution Guidelines

• I confirm that I have read the contribution guidelines and comply with them.

[per1234]

Suggested change

	If your Custom Brick requires external services, you can define them using a Docker Compose file. The system will automatically pull and run these containers alongside your App.
	If your Custom Brick requires external services, you can define them in this file. `brick_compose.yaml` is a Docker Compose file:
	https://docs.docker.com/reference/compose-file/
	The system will automatically pull and run these containers alongside your App.

• Adjust wording to make it clear that brick_compose.yaml is the referenced Docker Compose file.
• Link to the essential high quality Docker Compose file documentation

[per1234]

Suggested change

	image: custom_registry/my_service:latest
	image: some_registry/some_image:latest

• Adjust placeholder reference components to avoid giving the false impression that use of an image created by the user is required. Existing containers may also be utilized in custom Bricks.
• Use correct "image" term (reference).

[per1234]

Suggested change

	Once created, using a Custom Brick is identical to using a built-in Brick. Import the class from your package and initialize it in your App's `main.py`.
	Once created, using a Custom Brick is identical to using a built-in Brick. Import the class from the Python package in the Brick and initialize it in your App's `main.py`.

Adjust wording to make it more clear what is being referred by "your package".

[per1234]

This is certainly better than no documentation of the feature at all, so I'm fine with shipping the initial version of the docs with this as-is. However, I feel that it doesn't provide nearly enough information for the average user to be able to utilize the feature without reverse engineering how it is done in the built-in Bricks.