add ability to include custom javascript and css files.

[azjezz]

It would nice to allow developer to customize their documentation using javascript and css.

two new configuration entries would be needed for this:

1. scripts:

scripts:
  - 'path/to/my_script.js'

2. styles:

styles:
  - 'path/to/my_style.css'

If a path points to a file in the filesystem, a copy should be made to assets/, if it's a url
it should be used as is.

[azjezz]

this would allow people to include things like https://highlightjs.org to highlight code that is in the phpdoc, e.g:

<?php

namespace Useless;

/**
 * This function does nothing!
 * 
 * Example:
 *
 * ```php
 * use function Useless\foo;
 * 
 * foo(); // does absolutely nothing!
 * ```
 */
function foo(): void {}

[ondrejmirtes]

I think you can already do this. ApiGen has a theming ability. See how I was able to add a tracking code: phpstan/phpstan-src@c839382

Related issue: #1077

[azjezz]

nice! that solves my problem, but i think having this done dynamic in the config without creating a theme directory and having to manually copy the assets is better, so I'll keep this issue open.