Markdown cleanup/style

[romanc]

Description

This PR cleans up README.md and CONTRIBUTING.md with respect to markdown style. In particular we ensure

• consistent whitespace around headers and code blocks
• only fenced code blocks
• for shell code blocks, the $ sign implies that output is shown. Since there's no output, I removed them.

How Has This Been Tested?

I use markdownlint in VSCode to show violations of common markdown syntax/style.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas: N/A
• I have made corresponding changes to the documentation: N/A
• My changes generate no new warnings: N/A
• Any dependent changes have been merged and published in downstream modules: N/A
• New check tests, if applicable, are included: N/A

[romanc]

@fmalatino can't assign reviewers due to lacking permissions. Please have a look. Thanks!

[romanc]

@fmalatino thanks for approving. Please merge yourself. I don't have rights to merge in this repo. Thanks 🙏