fix: correct .htp typo to .hhp in HTMLHelpBuilder epilog

[VedantMadane]

Summary

This PR fixes a typo in the \HTMLHelpBuilder.epilog\ message.

Problem

The epilog message incorrectly states:

You can now run HTML Help Workshop with the .htp file in %(outdir)s.

However, HTML Help Workshop uses .hhp files as project files, not .htp files.

Fix

Changed .htp to .hhp in the epilog message.

Fixes sphinx-doc/sphinx#12308

[jdillard]

why the index.rst change?

[VedantMadane]

The index.rst change is from a follow-up commit (931a1c7): "fix: replace deprecated keyword index entry with pair to fix CI on Sphinx 9+".
The tests/roots/test-chm/index.rst fixture is used by the htmlhelp tests (including the ones validating .hhk output). The old bare keyword: with syntax is deprecated in Sphinx 9+ and was causing CI failures. I added the minimal normalization in a separate commit so the PR keeps the test suite green on current Sphinx.
Ready to split it into its own tiny PR if preferred, it was just the smallest change needed to make the htmlhelp tests pass while I was verifying the epilog fix.

[jdillard]

Ah I see, a separate PR has already been made for that https://github.com/sphinx-doc/sphinxcontrib-htmlhelp/pull/54/changes.

It looks like this PR a duplicate of #42 and can be closed.