Move extlinks into the main Sage code

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Split off from #25786

CC:  @saraedum @antonio-rojas

Component: **documentation**

Author: **Julian Rüth**

Branch/Commit: **[`13958db`](https://github.com/sagemath/sagetrac-mirror/commit/13958dbee41d1b3c7d4c74e2aacc8a4bdeba0216)**

Reviewer: **Jeroen Demeyer**

_Issue created by migration from https://trac.sagemath.org/ticket/25843_

