QA: add markdown workflow and configs

... to check the code style and various other things in the markdown stored in this repo.

Uses the previously created reusable workflows.

.github/workflows/markdown.yml

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+name: Markdown
+
+on:
+  # Run on all pushes and on all pull requests.
+  push:
+  pull_request:
+  # Allow manually triggering the workflow.
+  workflow_dispatch:
+
+jobs:
+  markdownlint:
+    name: 'Lint Markdown'
+    uses: ./.github/workflows/reusable-markdownlint.yml
+
+  remark:
+    name: 'QA Markdown'
+    uses: ./.github/workflows/reusable-remark.yml

.markdownlint-cli2.yaml

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+#
+# Configuration file for MarkdownLint-CLI2.
+#
+# Example file with all options:
+# https://github.com/DavidAnson/markdownlint-cli2/blob/main/test/markdownlint-cli2-yaml-example/.markdownlint-cli2.yaml
+#
+
+# Do not fix any fixable errors.
+fix: false
+
+# Define glob expressions to use (only valid at root).
+globs:
+  - "**/*.md"
+
+# Show found files on stdout (only valid at root)
+showFound: true
+
+# Define glob expressions to ignore.
+ignores:
+  - "node_modules/"
+
+# Disable inline config comments.
+noInlineConfig: true
+
+# Disable progress on stdout (only valid at root).
+noProgress: false
+
+# Adjust the configuration for some built-in rules.
+# For full information on the options and defaults, see:
+# https://github.com/DavidAnson/markdownlint/blob/main/schema/.markdownlint.yaml
+config:
+  ######################
+  # Disable a few rules.
+  ######################
+  # MD031/blanks-around-fences - Fenced code blocks should be surrounded by blank lines.
+  MD031: false
+  # MD032/blanks-around-lists - Lists should be surrounded by blank lines.
+  MD032: false
+
+  ##############################
+  # Customize a few other rules.
+  ##############################
+  # MD003/heading-style/header-style - Heading style.
+  MD003:
+    # Heading style - Always use hashes.
+    style: "atx"
+
+  # MD004/ul-style - Unordered list style.
+  MD004:
+    # List style - each level has a different, but consistent symbol.
+    style: "sublist"
+
+  # MD007/ul-indent - Unordered list indentation.
+  MD007:
+    indent: 4
+    # Whether to indent the first level of the list.
+    start_indented: false
+
+  # MD012/no-multiple-blanks - Multiple consecutive blank lines.
+  MD012:
+    maximum: 2
+
+  # MD013/line-length - Line length.
+  MD013:
+    # Number of characters. No need for being too fussy.
+    line_length: 1000
+    # Number of characters for headings.
+    heading_line_length: 100
+    # Number of characters for code blocks.
+    code_block_line_length: 100
+    # Stern length checking (applies to tables, code blocks etc which have their own max line length).
+    stern: true
+
+  # MD024/no-duplicate-heading/no-duplicate-header - Multiple headings with the same content.
+  MD024:
+    # Only check sibling headings.
+    siblings_only: true
+
+  # MD044/proper-names - Proper names should have the correct capitalization.
+  MD044:
+    # List of proper names.
+    names: ["PHPCSStandards"]
+    # Include code blocks.
+    code_blocks: false
+
+  # MD046/code-block-style - Code block style
+  MD046:
+    style: "fenced"
+
+  # MD048/code-fence-style - Code fence style
+  MD048:
+    style: "backtick"
+
+  # MD049/emphasis-style - Emphasis style should be consistent
+  MD049:
+    style: "underscore"
+
+  # MD050/strong-style - Strong style should be consistent
+  MD050:
+    style: "asterisk"

.remarkignore

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+# Ignore rules for Remark.
+# Docs: https://github.com/unifiedjs/unified-engine/blob/HEAD/doc/ignore.md
+
+/node_modules/

.remarkrc

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+{
+  "plugins": [
+    "remark-gfm",
+    ["remark-lint-checkbox-character-style", "consistent"],
+    ["remark-lint-checkbox-content-indent", "consistent"],
+    "remark-lint-definition-spacing",
+    "remark-lint-file-extension",
+    ["remark-lint-linebreak-style", "unix"],
+    ["remark-lint-link-title-style", "\""],
+    ["remark-lint-ordered-list-marker-style", "."],
+    "remark-lint-no-dead-urls",
+    "remark-lint-no-duplicate-defined-urls",
+    "remark-lint-no-duplicate-definitions",
+    "remark-lint-no-empty-url",
+    "remark-lint-no-file-name-consecutive-dashes",
+    ["remark-lint-no-file-name-irregular-characters", "\\.a-zA-Z0-9-_"],
+    "remark-lint-no-file-name-outer-dashes",
+    "remark-lint-no-heading-like-paragraph",
+    "remark-lint-no-literal-urls",
+    "remark-lint-no-reference-like-url",
+    "remark-lint-no-shortcut-reference-image",
+    "remark-lint-no-table-indentation",
+    "remark-lint-no-undefined-references",
+    "remark-lint-no-unneeded-full-reference-image",
+    "remark-lint-no-unneeded-full-reference-link",
+    "remark-lint-no-unused-definitions",
+    ["remark-lint-strikethrough-marker", "~~"],
+    ["remark-lint-table-cell-padding", "consistent"],
+    "remark-lint-heading-whitespace",
+    "remark-lint-list-item-punctuation",
+    "remark-lint-match-punctuation",
+    "remark-lint-no-hr-after-heading",
+    "remark-lint-are-links-valid-duplicate",
+    "remark-validate-links"
+  ]
+}