docs: migrate documentation over to mkdocs (#506)

A large migration of documentation over to mkdocs, and some rewrites. Some stuff (install information, basic supported systems, contributors, thanks) are still staying in README.md, and CONTRIBUTING.md is essentially duplicated right now. However, stuff like configuration and key/mouse bindings are now moved to mkdocs.

Some parts are still a bit WIP - it is definitely not done (documentation never seems to be...). However, it should be "good enough" for now, and I'm much happier working with the documentation in this form than trying to scroll through a giant endless README.md file. It also works much better for adding new documentation.

.github/workflows/docs.yml

@@ -0,0 +1,27 @@
+name: docs
+on:
+  workflow_dispatch:
+    # inputs:
+    #   version:
+    #     description: "The documentation version to update. Defaults to nightly."
+    #     default: "nightly"
+    #     required: false
+  push:
+    branches:
+      - master
+    paths:
+      - 'docs/**'
+      - '.github/workflows/docs.yml'
+jobs:
+  build-documentation:
+    name: Build and deploy docs
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-python@v2
+        with:
+          python-version: 3.x
+      - run: pip install mkdocs-material
+      - run: pip install mdx_truly_sane_lists
+      - run: cd docs/
+      - run: mkdocs gh-deploy --force

.github/workflows/nightly.yml

@@ -11,6 +11,7 @@ on:
       isMock:
         description: "Mock nightly run, fill in with anything to trigger a mock build."
         default: ""
+        required: false
 
 jobs:
   create-github-release: