A side effect of 71995a9087 caused the
dynamic table of contents in the left-hand side navigation to no longer
be displayed, likely due to the missing "details" DIV.
This commit addresses this issue by applying the custom header only to
index.adoc.
Consequently, the TOC is now displayed again on all pages except the
index page, but the customized header is no longer applied to those
pages.
We may revisit this issue if we decide that we want the custom header
on all pages (and not just the index page).
Closes gh-27177
Sam Brannen