docs: Add basic documentation system based on nmd

You can now run `nix build '.#docs'` to build HTML documentation
documenting every single option on nixvim! Fortunately, thanks to
the 'description' field, most options are already documented, but
there are still a fair few that need documenting.

I will be taking care of those in the next few days. When those are
done, I will find a way to automatically rebuild documentation on every
repo push, and also add a PR hook requiring documentation.

Additionally, I will try to find a way to have per-page plugin docs.

docs/plugins/colorschemes/base16.md

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+# base16
+Base16 is a set of colorschemes, so it comes with several themes you can choose from:
+
+ - 3024
+ - apathy
+ - ashes
+ - atelier-cave-light
+ - atelier-cave
+ - atelier-dune-light
+ - atelier-dune
+ - atelier-estuary-light
+ - atelier-estuary
+ - atelier-forest-light
+ - atelier-forest
+ - atelier-heath-light
+ - atelier-heath
+ - atelier-lakeside-light
+ - atelier-lakeside
+ - atelier-plateau-light
+ - atelier-plateau
+ - atelier-savanna-light
+ - atelier-savanna
+ - atelier-seaside-light
+ - atelier-seaside
+ - atelier-sulphurpool-light
+ - atelier-sulphurpool
+ - atlas
+ - bespin
+ - black-metal-bathory
+ - black-metal-burzum
+ - black-metal-dark-funeral
+ - black-metal-gorgoroth
+ - black-metal-immortal
+ - black-metal-khold
+ - black-metal-marduk
+ - black-metal-mayhem
+ - black-metal-nile
+ - black-metal-venom
+ - black-metal
+ - brewer
+ - bright
+ - brogrammer
+ - brushtrees-dark
+ - brushtrees
+ - chalk
+ - circus
+ - classic-dark
+ - classic-light
+ - codeschool
+ - cupcake
+ - cupertino
+ - darktooth
+ - default-dark
+ - default-light
+ - dracula
+ - eighties
+ - embers
+ - flat
+ - fruit-soda
+ - github
+ - google-dark
+ - google-light
+ - grayscale-dark
+ - grayscale-light
+ - greenscreen
+ - gruvbox-dark-hard
+ - gruvbox-dark-medium
+ - gruvbox-dark-pale
+ - gruvbox-dark-soft
+ - gruvbox-light-hard
+ - gruvbox-light-medium
+ - gruvbox-light-soft
+ - harmonic-dark
+ - harmonic-light
+ - heetch-light
+ - heetch
+ - helios
+ - hopscotch
+ - horizon-dark
+ - ia-dark
+ - ia-light
+ - icy
+ - irblack
+ - isotope
+ - macintosh
+ - marrakesh
+ - material-darker
+ - material-lighter
+ - material-palenight
+ - material
+ - material-vivid
+ - materia
+ - mellow-purple
+ - mexico-light
+ - mocha
+ - monokai
+ - nord
+ - oceanicnext
+ - ocean
+ - onedark
+ - one-light
+ - outrun-dark
+ - papercolor-dark
+ - papercolor-light
+ - paraiso
+ - phd
+ - pico
+ - pop
+ - porple
+ - railscasts
+ - rebecca
+ - seti
+ - shapeshifter
+ - snazzy
+ - solarflare
+ - solarized-dark
+ - solarized-light
+ - spacemacs
+ - summerfruit-dark
+ - summerfruit-light
+ - synth-midnight-dark
+ - tomorrow-night-eighties
+ - tomorrow-night
+ - tomorrow
+ - tube
+ - twilight
+ - unikitty-dark
+ - unikitty-light
+ - woodland
+ - xcode-dusk
+ - zenburn
+
+More information can be found on the official base16 website, [here](http://chriskempson.com/projects/base16/).
+
+## Options
+
+### `colorschemes.base16.enable`
+**Description**: Enables base16
+
+**Type**: `bool`
+
+### `colorschemes.base16.useTruecolor`
+**Description**: Whether to use truecolor for the colorschemes. If set to
+false, you'll need to set up base16 in your shell.
+
+**Type**: `bool`
+
+**Default**: `true`
+
+### `colorschemes.base16.setUpBar`
+**Description**: Whether to install the matching plugin for your statusbar. This does nothing as of yet, waiting for upstream support.
+
+**Type**: `bool`
+
+**Default**: `true`