docs: introduce code-block highlight

[KisaragiEffective]

What have been done in this PR

• This converts indented code blocks to fenced style¹.
• This adds info-string² to improve accessibility. The README does not specify where to type the commands. If they have info-string, it will make easier to understand how to build.

Footnotes

1. It is required to add info-string, indented one can't have info-string². ↩

2. An info-string is used to determine how it will be highlighted by GitHub Flavored Markdown (calling GFM) parser. ↩ ↩²

[CLAassistant]

All committers have signed the CLA.

[aritger]

Thanks for the suggestion, but I'm happy enough with the current formatting of these commands in README.md, so I'm inclined to leave this as-is. I'm going to go ahead and close, but let me know if there is some benefit to code-block highlight (sorry, I don't know github's markdown that well, so I could be missing something).

[KisaragiEffective]

@aritger
Thanks for your review! I've updated the description to explain benefits. I'm glad if you would re-consider this PR, thank you.