4a317893

Remove `DocEmpty` from `Doc`

This makes the `Doc` structure a little more correct, eliminating cases like
`DocParagraph DocEmpty` and `DocAppend DocEmpty …` (which `docAppend` avoided,
but could still be constructed).

Instead it uses `Maybe Doc`, with `Nothing` as the empty `Doc`.

This also replaces `docAppend` with a `Semigroup` instance (which allows the use
of the `Maybe` monoid).

Agent Instructions

Each run corresponds to one flake attribute. Click a run for its logs and status.

Example: https://nix-ci.com/gh:sellout:pollock/drop-DocEmpty/4a31789359998615fd5234df8d82167ddf4369ca/366f7166-5cdb-4691-a441-296482409c4c

Wait for suite to complete:

curl --netrc -N https://nix-ci.com/gh:sellout:pollock/drop-DocEmpty/4a31789359998615fd5234df8d82167ddf4369ca/wait

This page is also available in machine-friendly formats. Request one with the Accept header:

• curl --netrc -H 'Accept: application/json' https://nix-ci.com/gh:sellout:pollock/drop-DocEmpty/4a31789359998615fd5234df8d82167ddf4369ca
• curl --netrc -H 'Accept: text/markdown' https://nix-ci.com/gh:sellout:pollock/drop-DocEmpty/4a31789359998615fd5234df8d82167ddf4369ca

For more instructions, fetch https://nix-ci.com/instructions .