externals: Add proxy wiki pages
externals: Add proxy wiki pages

Adding two wiki pages to our externals, so they're on the website and indexed for easy searching.

- https://github.com/cockpit-project/cockpit/wiki/Proxying-Cockpit-over-NGINX
- https://github.com/cockpit-project/cockpit/wiki/Proxying-Cockpit-over-Apache-with-LetsEncrypt

Fixes #525
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