After a discussion on emacs-devel[1] and a discussion on a
bug-report[2], this was commited on the master-branch:

    756daa93b3ef7ce33e741ab30000fa397fcd9783
    Author:     Mekeor Melire <mekeor@posteo.de>
    AuthorDate: Mon Dec 4 16:37:37 2023 +0100
    Commit:     Eli Zaretskii <eliz@gnu.org>
    CommitDate: Sat Jan 27 12:18:17 2024 +0200

    Add option Info-url-alist

The new option Info-url-alist allow users to add custom mappings from
manual-names to URLs. When the URL-SPEC is provided as string, users may
use the %e format-sequence. But in the most common case, namely when
URLs need to use a .html-suffix, this is not usable:

In the case of the special "Top" node, the %e format-sequence resolves
to "", i.e. the empty string. It is thus not possible to use a URL-SPEC
like "https://example.com/%e.html" because "https://example.com/.html"
is invalid. The user rather wants "https://example.com/index.html" in
this case.

Currently, the user needs to choose to use a function as URL-SPEC in
this case. This works but is tedious, especially since this is the most
common case.

Thus, I suggest to provide another format-sequence to the
URL-SPEC-string: %i. It will resolve the "Top" node as "index", thus
allowing for "https://example.com/%i.html" to be used as URL-SPEC.

Find attached a patch that does so, and also refills the doc-string.


[1]  https://lists.gnu.org/archive/html/emacs-devel/2023-11/msg01286.html

[2]  https://lists.gnu.org/archive/html/bug-gnu-emacs/2023-12/msg00156.html