Version 2.1.0

This is Pico 2.1 - small, but mighty! 🎉 🎊

If you want to upgrade to Pico 2.1, simply follow the usual upgrade instructions for minor releases. Installing Pico is as easy as before. You can find more extensive upgrade instructions as well as a complete list of all additions and changes in Pico's upgrade docs - even though this is a minor release, it's still a lot of new and improved stuff! 💓

You might also want to give Pico CMS for Nextcloud 1.0 a try - it's now an official part of Pico. You can find more info at picocms.org.

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For a complete list of all changes compared to Pico 2.0, please refer to Pico's CHANGELOG.md (including the beta release v2.1.0-beta.1).

* [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