externals: Add catch2 v3.2.1

Merge commit '6879e5bb1c' as 'externals/catch'

externals/catch/docs/commercial-users.md

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+<a id="top"></a>
+# Commercial users of Catch2
+
+Catch2 is also widely used in proprietary code bases. This page contains
+some of them that are willing to share this information.
+
+If you want to add your organisation, please check that there is no issue
+with you sharing this fact.
+
+ - Bloomberg
+ - [Bloomlife](https://bloomlife.com)
+ - [Inscopix Inc.](https://www.inscopix.com/)
+ - Locksley.CZ
+ - [Makimo](https://makimo.pl/)
+ - NASA
+ - [Nexus Software Systems](https://nexwebsites.com)
+ - [UX3D](https://ux3d.io)
+ - [King](https://king.com)
+
+
+---
+
+[Home](Readme.md#top)