Add Boot Environment [Snapshots] documentation

[Monviech]

Important notices

Before you add a new report, we ask you kindly to acknowledge the following:

• [x] I have read the contributing guide lines at https://github.com/opnsense/docs/blob/master/CONTRIBUTING.md
• [x] I have searched the existing issues and I am convinced that mine is new.

Is your feature request related to a problem? Please describe.

Documentation for: https://github.com/opnsense/core/pull/7749

Describe the solution you like

• Short explanation about what boot Environments are
• How to create a Snapshot
• How to restore a Snapshot
• Usecases (like creating a Snapshot before doing a Major Upgrade)

Describe alternatives you considered

N/A

Additional context

N/A