Sync back remaining docs changes from Microsoft docs

[bergmeister]

PR Summary

As I was looking at creating a PR to bring docs change from PSSA repo to Microsoft docs repo, I noticed some changes at Microsoft docs weren't synced to PSSA repo, meaning I cannot just do a straightforward copy paste from PSSA to Microsoft docs and open PR. I identified the following docs changed that need backporting to PSSA repo first

• PR 79 with change to Get-ScriptAnalyzerRule.md https://github.com/MicrosoftDocs/PowerShell-Docs-Modules/pull/79/files#diff-407eadfb1309c36f5e4e63b23ae248c6a76a83aa28ac4cf6c40ce5ab1b536cb4
• PR 39 with change to overview.md, which was taken from PSSA's main README.md https://github.com/MicrosoftDocs/PowerShell-Docs-Modules/commit/278c15a5c40581a85ec7f54d891d048f2ad35077#diff-2b5005e2e817397[…]c837fa55f38e4d29f5f8
• PR 55 with changes to ShouldProcess.md https://github.com/MicrosoftDocs/PowerShell-Docs-Modules/pull/55/files

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Change is not breaking
• Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
• Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
• This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: to the beginning of the title and remove the prefix when the PR is ready.

[sdwheeler]

LGTM