Document new Wasm plugin system

[paigecalvert]

The new plugin system needs to be documented in the Helm 4 docs so that users know what it is, why it's great, how to migrate their existing plugins to the new system, and how to create their own plugins.

To-do

• Create a new folder under docs. For example, docs/plugins. Note that the folder name will be used in the URL path
• Create an index.mdx file in the new folder with this content docs/plugins/index.mdx:

  ---
  title: Helm Plugins # this title shows up in the sidebar. This section might include info about legacy and wasm plugins, so title should reflect that
  description: The section includes information about working with the Wasm plugin system. # edit this description as needed
  sidebar_position: <integer> # determines sidebar order. consider putting this under the Chart Templates Guide section 
  ---
  
  import DocCardList from '@theme/DocCardList';
  
  <DocCardList />
  

• Add the necessary content pages in this new section. Here are some recommendations (may or may not need all of these):
  • Overview of Wasm Plugins (docs/plugins/overview.md). What/why overview content
  • Migrate v3 Plugins to Wasm (docs/plugins/migrate.md). How to migrate existing plugins. The Helm 4 Migration Guide (what it's created) should link to this page.
  • How to Create Wasm Plugins (docs/plugins/create.md). Step-by-step how to create your own plugins. Could include a subheading with links to the Examples here
  • Wasm Plugin Spec/Reference (docs/plugins/reference.md). Is there any reference material that users might need when creating plugins? Maybe not needed if linking to third party docs covers it
• Move the https://helm.sh/docs/topics/plugins page to this new Plugins section and update accordingly to clarify that it's v3 (legacy plugins) specific

Related PRs and issues

• HIP: https://github.com/helm/community/blob/main/hips/hip-0026.md
• Tracking issue: [HIP-0026] implementation tracking issue (Helm 4 plugin system) helm#31144

[paigecalvert]

• See New Plugins section in Helm 4 docs #1899