Migrate setup, breaking changes, and known issues docs from source repo

[sdwheeler]

Migrate the following docs from the source repo into the docs repo.

• https://github.com/PowerShell/PowerShell/blob/master/docs/installation/linux.md
• https://github.com/PowerShell/PowerShell/blob/master/docs/installation/macos.md
• https://github.com/PowerShell/PowerShell/blob/master/docs/installation/windows.md
• https://github.com/PowerShell/PowerShell/blob/master/docs/BREAKINGCHANGES.md
• https://github.com/PowerShell/PowerShell/blob/master/docs/KNOWNISSUES.md

[sdwheeler]

The files have been moved to the PowerShell/PowerShell-Docs repo in the following locations:

• reference/docs-conceptual/setup/Installing-PowerShell-Core-on-Linux.md
• reference/docs-conceptual/setup/Installing-PowerShell-Core-on-macOS.md
• reference/docs-conceptual/setup/Installing-PowerShell-Core-on-Windows.md
• reference/docs-conceptual/whats-new/breaking-changes-ps6.md
• reference/docs-conceptual/whats-new/known-issues-ps6.md

[sdwheeler]

Submitted PR to remove docs from the source repo. PowerShell/PowerShell#6899

[sdwheeler]

Done