Relating to IAv3 and Redocly migration work, this moves introduction
files to match their location in the IA, so that their "pretty" URLs
under Redocly will also match.
The new folder will be compatible with both Dactyl and Redocly when it
comes to loading the Japanese translation for a given page.
Snippets are not moved for now because importing them would require
more complex changes to the actual Markdown pages, and wouldn't be
cross-compatible anyway (Redocly uses slightly different 'partial'
rather than 'include' syntax.)
