doc: fence code blocks

[silverwind]

Currently, we use 4-space indent to mark code blocks in the docs. It'd be better if we could replace them with backtick fences like js` or c++`. This has two benefits:

1. It's required for syntax highlighting to work on GitHub
2. It allows the code samples to be linted with tools like eslint-plugin-markdown

Because this would be another "churn" commit, I'd like to ask for opinions first.

[Fishrock123]

It's docs, churn doesn't exist like that imo

[eljefedelrodeodeljefe]

I agree. Also it's clearer while editing the docs for the eye, imo, and for markdown syntax highlighters. I'd offer some time to do it. Are there any blockers?

[silverwind]

I'd say go for it. One thing that needs to be checked beforehand is that the HTML output of make doc is showing correctly highlighted code blocks with the fences in place.

[eljefedelrodeodeljefe]

Alright. Will likely send in feedback today.

[eljefedelrodeodeljefe]

@silverwind I added three commits. The code block change is really big of course. I wanted to take some time on a bigger screen to review it tomorrow. I am pretty sure everything is alright though. Do have a look at the commit message for known issues.

I also added one to rename .markdown files, which has implications on tools and the makefile. Please do regard this as optional. I think the only reason no-one has changed this, is because it requires the change in 4 places.

[eljefedelrodeodeljefe]

@silverwind if you like, do have a look at Rods remarks on the PR. If you want to take this further, I am happy to change stuff accordingly.