Simplify documentation and configuration file CI system

Now that the parser tool is moved out of the repository, it makes less sense to use the taskfile-based approach for the
CI infrastructure. In order to make the repository more contributor-friendly, the documentation and configuration
checking system is now confined to the .github subfolder.

Author: per1234

.github/.markdown-link-check.json

@@ -0,0 +1,5 @@
+{
+  "retryOn429": true,
+  "retryCount": 3,
+  "aliveStatusCodes": [200, 206]
+}

.github/.markdownlint.yml

@@ -0,0 +1,61 @@
+# See: https://github.com/DavidAnson/markdownlint/blob/main/doc/Rules.md
+# The code style defined in this file is the official standardized style to be used in all Arduino projects and should
+# not be modified.
+# Note: Rules disabled solely because they are redundant to Prettier are marked with a "Prettier" comment.
+
+default: false
+MD001: false
+MD002: false
+MD003: false # Prettier
+MD004: false # Prettier
+MD005: false # Prettier
+MD006: false # Prettier
+MD007: false # Prettier
+MD008: false # Prettier
+MD009:
+  br_spaces: 0
+  strict: true
+  list_item_empty_lines: false # Prettier
+MD010: false # Prettier
+MD011: true
+MD012: false # Prettier
+MD013: false
+MD014: false
+MD018: true
+MD019: false # Prettier
+MD020: true
+MD021: false # Prettier
+MD022: false # Prettier
+MD023: false # Prettier
+MD024: false
+MD025:
+  level: 1
+  front_matter_title: '^\s*"?title"?\s*[:=]'
+MD026: false
+MD027: false # Prettier
+MD028: false
+MD029:
+  style: one
+MD030:
+  ul_single: 1
+  ol_single: 1
+  ul_multi: 1
+  ol_multi: 1
+MD031: false # Prettier
+MD032: false # Prettier
+MD033: false
+MD034: false
+MD035: false # Prettier
+MD036: false
+MD037: true
+MD038: true
+MD039: true
+MD040: false
+MD041: false
+MD042: true
+MD043: false
+MD044: false
+MD045: true
+MD046:
+  style: fenced
+MD047: false # Prettier

.github/workflows/check-config.yml

@@ -0,0 +1,67 @@
+name: Check Markdown
+
+# See: https://docs.github.com/en/free-pro-team@latest/actions/reference/events-that-trigger-workflows
+on:
+  push:
+    paths:
+      - ".github/workflows/check-markdown.yml"
+      - ".github/.markdown-link-check.json"
+      - "**/.markdownlint*"
+      - "**.md"
+      - "**.mdx"
+      - "**.mkdn"
+      - "**.mdown"
+      - "**.markdown"
+  pull_request:
+    paths:
+      - ".github/workflows/check-markdown.yml"
+      - ".github/.markdown-link-check.json"
+      - "**/.markdownlint*"
+      - "**.md"
+      - "**.mdx"
+      - "**.mkdn"
+      - "**.mdown"
+      - "**.markdown"
+  schedule:
+    # Run every Tuesday at 8 AM UTC to catch breakage caused by changes to markdownlint.
+    - cron: "0 8 * * TUE"
+  workflow_dispatch:
+  repository_dispatch:
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      - name: Initialize markdownlint-cli problem matcher
+        uses: xt0rted/markdownlint-problem-matcher@v1
+
+      - name: Install markdownlint-cli
+        run: sudo npm install --global markdownlint-cli
+
+      - name: Run markdownlint
+        run: markdownlint --config "${{ github.workspace }}/.github/.markdownlint.yml" "**/*.md"
+
+  links:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      - name: Determine whether only modified files should be checked
+        id: check-modified
+        if: github.event_name == 'pull_request'
+        run: |
+          echo "::set-output name=value::yes"
+
+      - name: Check links
+        uses: gaurav-nelson/github-action-markdown-link-check@v1
+        with:
+          config-file: .github/.markdown-link-check.json
+          use-quiet-mode: "yes"
+          check-modified-files-only: ${{ steps.check-modified.outputs.value }}
+          base-branch: ${{ github.base_ref }}