fix invalid escape warning when building docs

[pmeier]

When building the docs, we can see the following warning

INFO    -  DeprecationWarning: invalid escape sequence \[
             File "/home/philip/git/ora/ragna/deploy/_cli/core.py", line 18, in <module>
               from .config import ConfigOption, check_config, init_config
             File "/home/philip/git/ora/ragna/deploy/_cli/config.py", line 216, in
               f"$ pip install 'ragna\[all]'"

This comes from

ragna/ragna/deploy/_cli/config.py

Lines 211 to 217 in 8aa6bd0

	rich.print(
	f"\nTo do this, you can run\n\n"
	f"$ pip install {' '.join(unmet_package_requirements)}\n\n"
	f"Optionally, you can also install Ragna with all optional dependencies"
	f"for the builtin components\n\n"
	f"$ pip install 'ragna\[all]'"
	)

We need to escape the backslash there, since rich would otherwise interpret the [all] as markup.

This PR uses the rich functionality to escape instead. This gives the same result for rich, but also resolves the warning.

[pavithraes]

LGTM :)