`limactl help` should explain the limitation of the YQ expression support

[AkihiroSuda]

• Disable env access and file load operations in yqlib #4405 (comment)

[...]

• Add a document to explain our "dialect" of yq language

I'm struggling to find a good place in the documentation for this. Like a place users would actually find.

I feel like we also need more documentation for the create --set option, including examples. This could go in the limactl create --help output, and could then mention the limitation on env and file operators in our YQ dialect. Other options could then point to it.

This might still be a bit unwieldy, so maybe we should start having "concept guides" the way git does it:

❯ git help -g
The Git concept guides are:
   core-tutorial    A Git core tutorial for developers
   credentials      Providing usernames and passwords to Git
   cvs-migration    Git for CVS users
   diffcore         Tweaking diff output
   everyday         A useful minimum set of commands for Everyday Git
   faq              Frequently asked questions about using Git
   glossary         A Git Glossary
   namespaces       Git namespaces
   remote-helpers   Helper programs to interact with remote repositories
   submodules       Mounting one repository inside another
   tutorial         A tutorial introduction to Git
   tutorial-2       A tutorial introduction to Git: part two
   workflows        An overview of recommended workflows with Git

'git help -a' and 'git help -g' list available subcommands and some
concept guides. See 'git help <command>' or 'git help <concept>'
to read about a specific subcommand or concept.
See 'git help git' for an overview of the system.

There is also kubectl options as another precedent; not sure if it has other meta commands.

Anyways, this is quickly becoming out-of-scope for this PR and should be in a separate docs PR.

Originally posted by @jandubois in #4405 (comment)

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• Disable env access and file load operations in yqlib #4405 (comment)

[...]

concept guides

This can be probably implemented using https://pkg.go.dev/github.com/spf13/cobra#Command.IsAdditionalHelpTopicCommand

Originally posted by @AkihiroSuda in #4405 (comment)