Overall documentation improvements

[boesing]

The versioning is only the first step, the structure also needs an update. At the moment I see the following problems:

• introduction is missing with descriptions for storage, patterns, etc.
• at the moment, the basic usage page is not included in the navigation
• available adapters are hidden
• the adapters page is too long

My suggestions:

• create an introduction
• rework the entire quick start page and add it to the navigation
• create a separate page for each adapter, then they can be listed in the navigation
• add descriptions for application integrations (laminas-mvc, Mezzio)
• extend the homepage and add the features of laminas-cache (available adapters, PSR support, etc.)

Originally posted by @froschdesign in #136 (comment)