* docs: change link of 'should' in index.md

* docs: shorten links of npm packages in index.md

- made footer responsive
- made matomo & openjsf logos responsive
- remove `target="_blank"` since people can decide if they want to open a tab

…4133)

Given a test such as:

```js
it('foobars', 4);
```

mocha used to throw the following error: `fn.call is not a function`.

While technically true, I have personally spent some minutes of my life chasing
what looked like a bug in the test itself, and not in the call to `it`. A more
descriptive error message helps bring attention to where the problem originates.

(Thanks to [ssube](https://github.com/ssube) for helping with the error wording.)

…4150)