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

polkadot/node/gum/README.md

@@ -52,6 +52,6 @@ when providing to any of the log macros (`warn!`, `info!`, etc.).
 The crate has to be used throughout the entire codebase to work consistently, to
 disambiguate, the prefix `gum::` is used.
 
-Feature parity with `tracing::{warn!,..}` is not desired. We want consistency
+Feature Parity with `tracing::{warn!,..}` is not desired. We want consistency
 more than anything. All currently used features _are_ supported with _gum_ as
 well.

polkadot/node/malus/README.md

@@ -18,7 +18,7 @@ defined in the [(DSL[(**D**omain **S**pecific **L**anguage)]) doc](https://parit
 
 ## Usage
 
-> Assumes you already gained permissiones, ping in element @javier:matrix.parity.io to get access.
+> Assumes you already gained permissiones, ping in element `@javier:matrix.parity.io` to get access.
 > and you have cloned the [zombienet][zombienet] repo.
 
 To launch a test case in the development cluster use (e.g. for the  ./node/malus/integrationtests/0001-dispute-valid-block.toml):
@@ -48,7 +48,7 @@ This will also teardown the namespace after completion.
 ## Container Image Building Note
 
 In order to build the container image you need to have the latest changes from
-polkadot and substrate master branches.
+Polkadot and Substrate master branches.
 
 ```sh
 pwd # run this from the current dir

polkadot/node/metrics/README.md

@@ -1,4 +1,4 @@
-# polkadot-node-metrics
+# `polkadot-node-metrics`
 
 ## Testing
 

polkadot/node/test/service/README.md

@@ -1,4 +1,4 @@
-# polkadot-test-service
+# `polkadot-test-service`
 
 ## Testing