Various corrections in the documentation (#5154)

An attempt to improve [the
docs](https://paritytech.github.io/polkadot-sdk/master/polkadot_sdk_docs/index.html)
by applying various corrections:

- grammar/stylistics,
- formatting,
- broken links,
- broken markdown table,
- outdated vscode setting name,
- typos,
- consistency,
- etc.

Part of https://github.com/paritytech/eng-automation/issues/10