[docs] Rename Theme tutorials to Plugin and Theme Tutorials

[marcysutton]

We're working on some new tutorials (##20527) as part of the plugin authoring workflow (#20232), and it's become clear that we need a place to put those new learning materials. Therefore, we want to rename Theme Tutorials to Plugin and Theme Tutorials. This is an intentional move to include authoring tutorials in the same area of the docs as "using Gatsby" tutorials. That means we can move two plugin authoring tutorials into this section from the docs (Pixabay images and Remark Plugins).

This issue will be satisfied when the following items have been completed:

• Rename the Theme Tutorials page title to Plugin and Theme Tutorials, and change the filename accordingly.
• Do a find/replace for /tutorial/theme-tutorials to /tutorial/plugin-and-theme-tutorials
• Move the 2 docs tutorials into the tutorials directory: Pixabay Source Image Tutorial and Remark Plugin Tutorial
  • Pixabay Image Tutorial: https://www.gatsbyjs.org/docs/pixabay-source-plugin-tutorial/
  • Remark Tutorial: https://www.gatsbyjs.org/docs/remark-plugin-tutorial/
• Do a find/replace for links to those tutorials to their new locations
• Add redirects for the above 3 pages in _redirects.yml

Docs renaming instructions: https://www.gatsbyjs.org/contributing/docs-contributions/#docs-renaming-instructions

[marcysutton]

Closed with #21908.