style: Add markdownlint configuration

Add configuration files for markdownlint [1] and the related
markdownlint-cli2 [2]. Note that while the markdownlint-cli2
configuration file could include the configuration for markdownlint, it
is kept in a separate file, because for .markdownlint.yaml there is a
schema file which enables autocompletion in the IDE.

Prefer [1] over [3] which provides a comparable rules set, because the
former can easily be run locally with `npx markdownlint-cli2` which is
similar to `commitlint`, while the latter requires a working Ruby
installation. Also, chose `markdownlint-cli2` over `markdownlint-cli`
[4] for the reasons explained by the author in [5].

[1]: https://github.com/DavidAnson/markdownlint
[2]: https://github.com/DavidAnson/markdownlint-cli2
[3]: https://github.com/markdownlint/markdownlint
[4]: https://github.com/igorshubovych/markdownlint-cli
[5]: https://dlaa.me/blog/post/markdownlintcli2

Signed-off-by: Martin Nonnenmacher <[email protected]>

.markdownlint-cli2.yaml

@@ -0,0 +1,11 @@
+#  Configuration for the markdownlint-cli2, see:
+#  https://github.com/DavidAnson/markdownlint-cli2/blob/main/test/markdownlint-cli2-yaml-example/.markdownlint-cli2.yaml
+
+globs:
+  - "**/*.md"
+ignores:
+  - "**/node_modules/**"
+  - "**/projects/external/**"
+  - "**/multi-kotlin-project/**"
+  - "**/sbt-http4s-template/**"
+  - "**/stack-yesodweb-simple/**"

.markdownlint.yaml

@@ -0,0 +1,35 @@
+# Markdownlint configuration, see: https://github.com/DavidAnson/markdownlint/blob/main/schema/.markdownlint.yaml
+# For the list of rules see: https://github.com/DavidAnson/markdownlint/blob/main/README.md#rules--aliases
+
+# ul-style
+MD004:
+  style: asterisk
+
+# line-length
+MD013:
+  code_blocks: false
+  line_length: 120
+  tables: false
+
+# no-duplicate-heading
+MD024: false # GitHub has no issues with anchors for headings with the same content.
+
+# no-inline-html
+MD033: false # Inline HTML is used for anchors in README.md where images are used in headings.
+
+# no-bare-urls
+MD034: false # GitHub has no issues with rendering bare URLs.
+
+# fenced-code-language
+MD040: false # It is not possible to always set a language, for example in case of config files using uncommon formats.
+
+# first-line-heading
+MD041: false # Some short README.md files do not require a heading.
+
+# code-block-style
+MD046:
+  style: fenced
+
+# emphasis-style
+MD049:
+  style: asterisk