Add help pages for each command

[BreD1810]

Description

• Added help pages for 'pipeline', 'mr', and 'issue' and their subcommands.
• Refactored help page for 'repo'
• Added support for the '-h' switch
• Fixed an issue where invalid command would be shown after the help page:

glab issue --help

Issue Help
help: Invalid Command

Related Issue
#44

Motivation and Context
This change provides help for how to use each command, and it's subcommands.

How Has This Been Tested?
Manually.

Screenshots (if appropriate):
For example:

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes. - N/A
• All new and existing tests passed. - No tests

[pull-assistant]

Best reviewed: commit by commit

---

Optimal code review plan

     feat(help): Add 'issue' help pages

     feat(help): Add 'mr' help pages

     Merge branch 'trunk' of https://github.com/profclems/glab into ft-help...

     feat(help): Add 'pipeline' help pages

     feat(help): Add 'repo' help pages

     feat(help): Add support for '-h' switch

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[profclems]

Great!