Write user documentation for Electrum

We might want to get inspiration from, or link to one of these:

• https://electrum.org/tutorials.html: Very bare bone tutorial (but see last paragraph).
• http://bitzuma.com/posts/a-beginners-guide-to-the-electrum-bitcoin-wallet/: Very verbose, and uses screenshots extensively. It starts with an “Installation” part, but the HTML document has anchors for each section, which makes it easier to link to specific parts we feel are relevant.

Feature Branch: feature/6739-install-electrum

Parent Task: #6739 (closed)

Original created by @anonym on 8163 (Redmine)