Version 2.1.0

```
* [Changed] Add Pico's official logo and tagline to `content-sample/_meta.md`
* [Changed] Improve `content-sample/theme.md` to show Pico's official logo and
            the usage of the new image utility classes of Pico's default theme
* [Changed] Improve Pico docs and PHPDoc class docs
```

Author: PhrozenByte