This commit reinstates the single-page variant for the Spring Boot
reference documentation.

Now the single and multi-page variants are hosted under the same
`"html"` folder, as `"index.html"` and `"single-index.html"`.
Note that files ending in `"index.html`" are treated as index files by
the spring-doc-resources project and their table of contents do not
display a "Back to index" link.

Fixes gh-16295