docs: move plugin configuration guides to plugins section

[DCchoudhury15]

What this PR does:
Moves plugin-specific configuration guides out of the User Guide and into the new Plugins section. Specifically:

• Extracts the Kubernetes and Terraform plugin configuration sections from user-guide/managing-piped/configuring-a-plugin.md into dedicated pages under plugins/
• Moves user-guide/managing-piped/adding-an-analysis-provider.md to plugins/analysis.md
• Updates plugins/_index.md to link to the three new pages instead of showing a WIP placeholder
• Adds a redirect note in configuring-a-plugin.md pointing users to the new Plugins section

Why we need it:
The plugin documentation was scattered across the User Guide. Having dedicated pages under the Plugins section makes it easier for users to find plugin-specific configuration guides in one place.

Which issue(s) this PR fixes:
Fixes #6540

Does this PR introduce a user-facing change?:

• How are users affected by this change: Plugin configuration guides are now located at /docs-v1.0.x/plugins/ instead of inside the User Guide. A note in configuring-a-plugin.md directs users to the new location.
• Is this breaking change: No
• How to migrate (if breaking change): N/A