gh-37598: Sphinx extlinks for ecl, gap, giac, maxima, meson, ppl, qepcad, scip, singular, soplex
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We create a standard way to refer to the online manuals of these
packages, which are generated by tools other than Sphinx and therefore
cannot be linked to using Intersphinx.
This extends what has been already done with Pari.
Examples:
- [Examples in the Developer Guide](https://deploy-preview-37598--
sagemath.netlify.app/html/en/developer/sage_manuals#hyperlinks)
Fixes #27495.
Outside of the scope of this PR:
- adding such extlinks everywhere
- working with upstream projects to upgrade to better documentation
tools
- projects that can be linked to using Intersphinx; that's #37575
- any consideration how to link instead to local copies of those
manuals, whether installed from SPKG or system packages.
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### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/37598
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Matthias Köppe, Max Horn
