Support LibreJS in HTML - machine readable license

[prosaole]

LibreJS (https://www.gnu.org/software/librejs/) is a plug-in that check if the Java-Script is free software before running it.

Sphinx generates Java-Script that is free software, but LibreJS cannot see that, and therefore treats this as non-free.

There are several ways to tell LibreJS the license, but the easiest is probably to add a line in each script like this:

// @license magnet:?xt=urn:btih:87f119ba0b429ba17a44b4bffcab33165ebdacc0&dn=freebsd.txt

(Full list of licenses https://www.gnu.org/software/librejs/manual/html_node/Setting-Your-JavaScript-Free.html#License-tags)

And while we are on the subject of machine readable licenses: To support https://reuse.software/ it might make sense to also add:

// SPDX-FileCopyrightText: 2024 Foo Bar
// SPDX-License-Identifier: BSD-2-Clause

See full list on https://spdx.org/licenses/

[picnixz]

PR is welcomed ! I think we can follow the guidelines that are suggested.