PG-473 Drop docs and disable build on GH pages

With the documentation moved to a separate repo, remove the docs from this repository to avoid confusion where to contribute to docs. Also, removing a GH action and unpublishing the GH pages site with the docs being built on Render. Updated README and Contributing guide with the new docs repo.

deleted:    .github/workflows/doc-build.yml
	modified:   README.md
	deleted:    docs/COMPARISON.md
	deleted:    docs/REFERENCE.md
	deleted:    docs/USER_GUIDE.md
	deleted:    docs/_images/percona-favicon.ico
	deleted:    docs/_images/percona-logo.svg
	deleted:    docs/css/percona.css
	deleted:    docs/css/toctree.css
	deleted:    docs/index.md
	deleted:    docs/js/version-select.js
	deleted:    docs/overrides/404.html
	deleted:    docs/setup.md
	deleted:    requirements-doc.txt

.github/workflows/doc-build.yml

@@ -1,50 +0,0 @@
-name: Build pg_stat_monitor docs
-on:
-  push:
-    branches: 
-      - REL_1_STABLE
-      
-      
-jobs:
-  build:
-    name: Deploy docs
-    runs-on: ubuntu-latest
-
-    steps: 
-      #Pull the latest changes
-      - name: Chekout code
-        uses: percona-platform/checkout@v2
-        with:
-          fetch-depth: 0
-      #Prepare the env
-      - name: Set up Python
-        uses: percona-platform/setup-python@v2
-        with:
-          python-version: '3.x'
-      
-      #Configure git
-      - name: Configure git
-        env:
-          ROBOT_TOKEN: ${{ secrets.ROBOT_TOKEN }}
-        run: |
-          git config --global url."https://percona-platform-robot:${ROBOT_TOKEN}@github.com".insteadOf "https://github.com"
-          git config user.name "GitHub Action"
-          git config user.email "github-action@users.noreply.github.com"
-          git config user.password "${ROBOT_TOKEN}"
-          echo "GIT_USER=percona-platform-robot:${ROBOT_TOKEN}" >> $GITHUB_ENV
-
-      #Set up MkDocs
-      - name: Setup MkDocs
-        run: |
-          python -m pip install --upgrade pip
-          pip install wheel
-          if [ -f requirements-doc.txt ]; then pip install -r requirements-doc.txt; fi
-
-     
-     # Deploy docs
-      - name: Deploy docs
-        run: |
-          mike deploy REL_1_STABLE -p
-          mike set-default REL_1_STABLE -p
-          mike retitle REL_1_STABLE  "1.0.x (stable)" -p
-