Move `pixldc` commands into PIXL CLI

[milanmlft]

Fixes #392

• **Add pixl dc CLI command as a wrapper around docker compose
• Remove pixldc
• Update docs

Periphery changes:

• Use common pixl-db envvars for orthanc-anon
• Remove obsolete cli._utils

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.87500% with 2 lines in your changes missing coverage. Please review.

Project coverage is 83.04%. Comparing base (1c356f2) to head (b6ebc33).

Files	Patch %	Lines
cli/src/pixl_cli/_docker_commands.py	93.93%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #411 +/- ## ========================================== - Coverage 83.26% 83.04% -0.22% ========================================== Files 78 76 -2 Lines 3268 3061 -207 ========================================== - Hits 2721 2542 -179 + Misses 547 519 -28 ```

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[milanmlft]

Reverted back to using docker compose directly in run-system-test.sh because attempting to use pixl up/down requires the following:

• They need to be run from the project root directory (though the exact reason for this is still vague to me)
• They require all of the environment variables in test/.env to be set, because running the CLI through Python results in a series of imports and config() calls, which will fail when the envvar is not set

There's a couple of workarounds for this, like temporarily copying over the test/.env file to project root, or loading the test/.env file through Python. But I'm going to leave it at this because this PR already took way too much time for what should have been a simple change 🫠

More importantly though, I think this points to some not-so-ideal design choices in the way we're handling these environment variables and .env files. Because when I'm running pixl down, I don't really care whether RABBITMQ_HOST is defined or not, but at the moment that will result in an error. Might be worth having a think about!