docs: initialise new docs with docusaurus (#460)

* docs: initialise new docs with docusaurus

* chore: sign off
Signed-off-by: izzy <me@insrt.uk>

* chore: lock pnpm version in package.json

Signed-off-by: izzy <me@insrt.uk>

* ci: specify package json to load pnpm version from

Signed-off-by: izzy <me@insrt.uk>

* docs: update repo. lifecycle details

Signed-off-by: izzy <me@insrt.uk>

* docs: add external links to for-web, for-android
chore: add default.nix to docs

* docs: correct headings in contrib

* ci: use cache dependency path

* fix: docusaurus configuration

---------

Signed-off-by: izzy <me@insrt.uk>

.github/workflows/docs.yml

@@ -0,0 +1,54 @@
+name: Documentation
+
+on:
+  push:
+    branches:
+      - main
+
+jobs:
+  build:
+    name: Build Docusaurus
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: ./docs
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+      - uses: pnpm/action-setup@v4
+        with:
+          package_json_file: ./docs/package.json
+      - uses: actions/setup-node@v4
+        with:
+          node-version: lts/*
+          cache: pnpm
+          cache-dependency-path: ./docs/pnpm-lock.yaml
+
+      - name: Install dependencies
+        run: pnpm install
+      - name: Build website
+        run: pnpm run build
+
+      - name: Upload Build Artifact
+        uses: actions/upload-pages-artifact@v3
+        with:
+          path: build
+
+  deploy:
+    name: Deploy to GitHub Pages
+    needs: build
+
+    permissions:
+      pages: write # to deploy to Pages
+      id-token: write # to verify the deployment originates from an appropriate source
+
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+
+    runs-on: ubuntu-latest
+    steps:
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v4