Add code of conduct from Alpine

As discussed in the team meeting, adopt Alpine's code of conduct. Put it on the homepage to make it official.

Changes:

• add the /coc page
• add a button at the bottom that links to this page
• camel case for all link labels at the bottom
• clarify URL in README.md of imported icons

Screenshot

content/page/coc.md

+title: "Code of Conduct"
+---
+The [Alpine Linux Code of Conduct](https://www.alpinelinux.org/community/code-of-conduct.html)
+is followed in postmarketOS.
+
+Alpine is the distribution we are based on, and
+our communities work closely together. The CoC is reasonable, it boils down to
+using your brain and being nice to each other. It is what we have been living
+since the project was started, for example people who were rude towards others
+have been kicked from chats.
+
+It helps to have this written down in detail, and when interacting with Alpine
+you should be aware of this anyway. So it made sense to adopt this CoC for
+postmarketOS.