Move taxonomy and terms out of _default

[chalin]

• Contributes to Adapt to new template system in Hugo v0.146.0 #2243
• Moves the final files out of layouts/_default to the top level of layouts
• Swaps files names. For details, see:
  • Layout taxonomy.html used before terms.html after moving out of _default gohugoio/hugo#13728
  • Taxonomy.html and terms.html reversed
• There don't seem to be any necessary UG changes.
• Use the links in the section below for previews

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EDIT: Solution found by swapping layouts
This currently breaks the UG taxonomies pages

For example, compare the taxonomy index page (called categories in Docsy):

• https://www.docsy.dev/categories/
• https://deploy-preview-2257--docsydocs.netlify.app/categories/

Also, the specific taxonomy page no longer renders any content:

• https://deploy-preview-2257--docsydocs.netlify.app/categories/taxonomies/
• Compare with - https://www.docsy.dev/categories/taxonomies/