docs: initial keys spec (#3035)

Initial keys requirements and draft spec which attempts to meet those
requirements (with some open questions).

It's not a polished doc, yet. It's a WIP that can be iterated on in the
coming weeks.

**Important Note**

You should definitely read the _rendered_ version of this.
Unfortunately, docusaurus uses KaTeX, but vscode/github use something
like MathJax, and they're annoyingly incompatible, in terms of the
syntax I need to write. It's docusaurus that's the odd one out here. My
latex vscode plugin/hackmd/github are all able to render one type of
syntax, but I had to modify the latex all over the place to render it in
a docusaurus page. So if you try to read it any other way, it'll be
borked.

**So view this thing using docusaurus, in your browser.**:
`cd yellow-paper`  
`yarn`
`yarn start:dev` (if on the mainframe)
Go to localhost:3000

yellow-paper/docs/addresses-and-keys/_category_.json

@@ -0,0 +1,8 @@
+{
+  "label": "Addresses & Keys",
+  "position": 2,
+  "link": {
+    "type": "generated-index",
+    "description": "addresses & keys..."
+  }
+}