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

[DCchoudhury15]

@eeshaanSA if no one is working on this right now , can i contribute?

[lavanyayadawad30-lab]

I want to work on this issue, @eeshaanSA could you assign this to me

[eeshaanSA]

Yes, sure. @DCchoudhury15, since you were first, do you want to work on this?

[DCchoudhury15]

Yes @eeshaanSA i will start working upon it , thanks for the reply

[riahtgl]

The issue has been assigned to you, @DCchoudhury15. Please give updates on how it's going. Thank you!

[DCchoudhury15]

@eeshaanSA I've started putting together a branch for this. Just wanted to sync up before I open a PR.

So far I've:

• Extracted the Kubernetes and Terraform sections from user-guide/managing-piped/configuring-a-plugin.md into their own files under plugins/.
• Moved adding-an-analysis-provider.md over to plugins/analysis.md (and fixed the relative links).
• Updated plugins/_index.md to link to these new docs instead of the WIP message.

I saw the issue mentioned moving "and more", but I did a quick search through the user-guide folder and couldn't find any other dedicated plugin guides (like ECS, Cloud Run, Wait, etc.).

Did I miss any hiding somewhere else, or should I just go ahead and open the PR with these three?