Docs v1: add plugins overview page

[rahulshendre]

What this PR does:
Fills in the plugins overview page

Why we need it:
The plugins section is the entry point for plugin docs in v1 and has been empty.
Plugin pages for each are coming in through #6698, #6712 and #6878 - once those merge, the plugin names in the tables can link to their pages.

Which issue(s) this PR fixes:

Part of #5615

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 49.37%. Comparing base (1ecc5d5) to head (f52377f).

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #6984       +/-   ##
===========================================
+ Coverage   29.37%   49.37%   +19.99%     
===========================================
  Files         601       51      -550     
  Lines       64276     5659    -58617     
===========================================
- Hits        18884     2794    -16090     
+ Misses      43924     2639    -41285     
+ Partials     1468      226     -1242     

Flag	Coverage Δ
.	?
.-pkg-app-pipedv1-plugin-analysis	?
.-pkg-app-pipedv1-plugin-ecs	?
.-pkg-app-pipedv1-plugin-kubernetes	58.37% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes_multicluster	?
.-pkg-app-pipedv1-plugin-scriptrun	54.83% <ø> (ø)
.-pkg-app-pipedv1-plugin-terraform	?
.-pkg-app-pipedv1-plugin-wait	?
.-pkg-app-pipedv1-plugin-waitapproval	?
.-pkg-plugin-sdk	?
.-tool-actions-gh-release	19.23% <ø> (ø)
.-tool-actions-plan-preview	25.51% <ø> (ø)
.-tool-codegen-protoc-gen-auth	?

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[mohammedfirdouss]

LGTM @rahulshendre

[github-actions]

Thank you for contributing to PipeCD, @rahulshendre! The changes in this pull request will be part of the upcoming release!