doc: Add VSCode color customization tip

[LemmusLemmus]

This PR simply adds a tip in the description of the VSCode documentation page, that the colors can be overridden using the VSCode settings.

Feel free to suggest rewording or reject this PR if it feels unnecessary. I personally first went looking at the docs, then Nix source code, to see if there was any option I could override, when the VSCode settings solution is surprisingly simple.

(I personally simply wanted to change the wordHighlightBackground and hoverHighlightBackground colors, since they are the same color as the normal text selection color and thus I could not see which part of the word I have selected).

Things done

• [x] Tested locally
• [ ] Tested in testbed
• [x] Commit message follows commit convention
• [ ] Fits style guide
• [x] Respects license of any existing code used

Notify maintainers

@Flameopathic @danth