Move Emacs documents to //emacs

These documents outline some of my Elisp conventions and ideas. As such,
I think //emacs should host them.

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+# Elisp Conventions
+
+Some of this aligns with existing style guides. Some of it does not.
+
+In general, prefer functions with fixed arities instead of variadic
+alternatives.
+
+- Namespace functions with `namespace/function-name`
+- Use `ensure`, `assert`, `refute` whenever possible.
+- When talking about encoding and decoding, let's use the words "encoding" and
+  "decoding" rather than the myriad of other variants that appear like:
+  - `marshalling` and `unmarshalling`
+  - `parse` and `deparse`, `serialize`, `stringify`
+  - `unpickle` and `pickle` (Python)
+  - `from-string` and `to-string`
+  - TODO: Add more examples of these; there should be close to a dozen.
+- Annotate assertions with `!` endings.
+- Prefer the Scheme style of `predicate?`
+- Variadic functions *should* encode this by appending * onto their
+  name. E.g. `maybe/nil?*`