Create ci.yml

[bsdelf]

Use GitHub Action to run CI, all CI does is lint markdown files with mdl command, see https://github.com/markdownlint/markdownlint project.

Currently, the default rules of mdl already found some issues in bunch of files:

% mdl * | grep "MD\d\+.*$" -o | sort | uniq
MD001 Header levels should only increment by one level at a time
MD004 Unordered list style
MD009 Trailing spaces
MD012 Multiple consecutive blank lines
MD025 Multiple top level headers in the same document
MD029 Ordered list item prefix
MD030 Spaces after list markers
MD032 Lists should be surrounded by blank lines
MD036 Emphasis used instead of a header

Most of the default rules are fine, except:

• MD004: :dash should be preferred, because - only needs a single key stroke while * requires shift
• MD029: it's hard to say which one is better, but :one definitely makes life easier.

I suggest we should discuess about the rules, once aligned I can add a config file for mdl and fix remaining lint issues in PR.

[bobby285271]

Just a note that with the new file structure introduced in b15b22e, mdl no longer tries to lint markdown files other than README.md due to the current wildcard *.md. While I believe simply changing *.md to . should fix this, pull requests are welcomed if you have better solutions.