docs: fix documentation index for hypercalls

docs: fix documentation index for hypercalls

This was broken by 118104e5eaf2 "docs: Build docs for ARM as well as x86_64".

Move docs to hypercall/ARCH instead of hypercall-ARCH.

Support mulitple levels of subdirectories in gen-html-index tool.

This removes the need for a symlink hypercall->hypercall-x86_64 since there is
now a proper index at hypercall/index.html.

Update INDEX to human readable names for the architecture specific hypercalls.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>