Add linting information to CONTRIBUTING.md

This adds information on how the docs use linting, and more detailed
instructions on how to enable it in an IDE, using VSCode as a concrete
and widespread example.

Additionally, to do this, we add a duplicate style config for
markdownlint in JSON format for the Node.js implementation of
markdownlint that is used by various editor plugins. These both
implement the same core set of warnings, and appear to function
identically.

Dockerfile

@@ -82,7 +82,7 @@ FROM gem AS mdl-base
 ARG MDL_JSON
 ARG MDL_STYLE
 RUN --mount=type=bind,target=. <<EOF
-  mdl --ignore-front-matter ${MDL_JSON:+'--json'} --style=${MDL_STYLE:-'.mdlrc.style.rb'} $( \
+  mdl --ignore-front-matter ${MDL_JSON:+'--json'} --style=${MDL_STYLE:-'.markdownlint.rb'} $( \
     find '.' -name '*.md' \
       -not -path './registry/*' \
       -not -path './desktop/extensions-sdk/*' \