fix(docs): fix broken links in Creating a Generic Plugin guide

[jsxs0]

Description

Fixed broken documentation links in the "Creating a Generic Plugin" guide. Updated all links to follow the current Gatsby v5 documentation structure under /docs/how-to/plugins-and-themes/.

Fixes #39210

Changes

• Updated /docs/creating-a-local-plugin/ → /docs/how-to/plugins-and-themes/creating-a-local-plugin/
• Updated /docs/what-is-a-plugin/ → /docs/how-to/plugins-and-themes/what-is-a-plugin/
• Updated /docs/files-gatsby-looks-for-in-a-plugin → /docs/how-to/plugins-and-themes/files-gatsby-looks-for-in-a-plugin/

Related Issues

Closes #39210

Testing Instructions

1. Visit the updated documentation page
2. Verify all links in the document work correctly
3. Specifically test the previously broken links:
   • Link to "Creating a Local Plugin"
   • Link to "What is a Plugin"
   • Link to "Files Gatsby Looks for in a Plugin"

[serhalp]

Thanks!