Improve plugin and theme usage docs

[PhrozenByte]

Provide step-by-step instructions on how to "install" plugins and themes in Pico.

• Either upload a single plugin file directly to the plugins/ directory or upload multiple plugin files into a to-be-created directory named exactly after the plugin
• Upload themes to the themes/ directory
• Explain that some plugins and themes don't just consist of the actual plugin/theme files, but rather also include themes or content files which should be uploaded to the appropriate Pico directory
• Firmly refer to the README.md of the plugin/theme

However: Keep it short! Users don't want to read pages over pages just to learn how to install a plugin/theme... It's dead simple. This shouldn't require much text.