docs: add css for inline code

[sh-cho]

Recently I am building a parser with tree-sitter and while reading the docs, I thought it would be better if the inline code was more readable, so I changed the style.

What do you think? I'd love to hear your thoughts.

Here are some examples of the styling

as-is	to-be

ex) https://tree-sitter.github.io/tree-sitter/creating-parsers#keywords

as-is	to-be

ex) https://tree-sitter.github.io/tree-sitter/creating-parsers#external-scanners

You can check other pages here: https://sh-cho.github.io/tree-sitter/

I only highlighted <code> inside #main-content except <code> inside <a> or <pre> tag.

[amaanq]

this is really nice, sorry for not getting to this sooner. thank you!