Markdown linter (#1309)

* Add markdown linting

- add linter default rules
- adapt rules to current code
- fix the code for linting to pass
- add CI check

fix #1243

* Fix markdown for Substrate
* Fix tooling install
* Fix workflow
* Add documentation
* Remove trailing spaces
* Update .github/.markdownlint.yaml

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
* Fix mangled markdown/lists
* Fix captalization issues on known words

docs/markdown_linting.md

@@ -0,0 +1,20 @@
+# Markdown linting
+
+Since the introduction of [PR #1309](https://github.com/paritytech/polkadot-sdk/pull/1309), the markdown
+files in this repository are checked by a linter for formatting and consistency.
+
+The linter used is [`markdownlint`](https://github.com/DavidAnson/markdownlint) and can be installed locally on your
+machine. It can also be setup as [pre-commit hook](https://github.com/igorshubovych/markdownlint-cli#use-with-pre-commit)
+to ensure that your markdown is passing all the tests.
+
+The rules in place are defined
+[here](https://github.com/paritytech/polkadot-sdk/blob/master/.github/.markdownlint.yaml).
+
+You may run `markdownlint` locally using:
+```
+markdownlint --config .github/.markdownlint.yaml --ignore target .
+```
+
+There are also plugins for your favorite editor, that can ensure that most
+of the rules will pass and fix typical issues (such as trailing spaces,
+missing eof new line, long lines, etc...)