Add documentation for beginners

[Potherca]

Based on feedback from a user, we need to add documentation for users that are beginners in the Solid landscape. This doesn't have to be exhaustive (for that links to other sources can be used) but should help people who do not know what solid is or how it works to get up and running with Solid on Nextcloud.

For example:

• What is a solid pod?
• What is a solid app?
• How / Where is my pod created?
• How to access a pod?

[almereyda]

In addition the installation instructions are right now a little confusing, as since few releases the available version for recent Nextcloud versions equals with the one released here. This means the current IMPORTANT notices on:

• https://apps.nextcloud.com/apps/solid
• https://github.com/pdsinterop/solid-nextcloud/blob/main/README.md

plus the depreciated rationale four source installs in:

• https://github.com/pdsinterop/solid-nextcloud/blob/main/INSTALL.md

could be adapted as well until the next planned 0.7.3 release.