Docs: Move certain sections from the User Guide to the new Plugins section

[eeshaanSA]

Review the v1 documentation user guide, and move some sections from the userguide to the plugins section, such as, how to use the k8s plugin, how to use the terraform plugin and more.

reference: https://pipecd.dev/docs-v1.0.x/user-guide/

[Jay2006sawant]

Hey @eeshaanSA Can I take up this issue?

[Shawn-Dong]

Dear @eeshaanSA , is this issue still available? I'd like to work on this. Thank you!

[eeshaanSA]

@Jay2006sawant, yes
@Shawn-Dong yes