Implement `pct help` subsystem

[sanfrancrisko]

Implement the pct help subsystem. This should provide:

• Overview of the command / subcommand
• A flag for documents by topic
• Links to online docs

Examples:

prm help --topic validating-bolt-tasks # Pulls up walkthru
prm help validate # Pulls up reference docs for prm validate
prm validate --help # Pulls up reference docs for prm validate
prm help set puppet # Pulls up reference docs for prm set puppet
prm help --topic backend # Concept doc for backends generally
prm help --topic backend-docker # Concept doc for configuring/using docker backend
prm help --tool rubocop # Documentation for the rubocop tool use via PRM, link to upstream tool docs

[michaeltlombardi]

I've done some investigation into custom help and cobra today with some useful findings to summarize here:

1. Command level help is automatic, so we don't actually need to do any substantial work for surfacing reference docs for commands; prm help set puppet will just work out of the box.
2. We can write additional subcommands such as prm exec describe puppetlabs/rubocop - with some shared code leveraged to make the same functionality available in validate.
3. I looked into the cobra concept of additional help commands; unfortunately, I think implementing something (with an example described here) would require metaprogramming to create these on the fly and runs the risk of clobbering the expected behavior as whether prm exec puppetlabs/rubocop --help should return the help as surfaced by the tool or the help about the tool package is not obvious. I suggest instead that we go with the direction in point 2 above for surfacing information about the tool package.
4. Adding flags to the base help command is possible but poor UX - calling prm help displays the help for the root prm command without any additional flags for help; those flags are visible if the user calls prm help help but this is incredibly non-obvious. So while prm help --topic foo is technically implementable I recommend we instead add an explain subcommand which can read docs by topic etc.

In summary:

• using the existing help subsystem to surface reference docs for commands is functional and useful
• we can have the commands for exec and validate include a describe subcommand which can surface the reference docs for that tool as packaged for PRM
• we can include an explain subcommand for surfacing topics written in markdown and sorted by topic/etc
• we can further investigate how we want to surface the actual tool's help information (ie rubocop --help in the container) via exec/validate without clobbering other information. Possibly simplest to do with a simple flag on the exec and validate commands.

[michaeltlombardi]

Resolved by https://github.com/puppetlabs/prm/pull/83