README.md: reduce content covered by docs

I noticed the current README.md has much overlap with the usage-section of the docs.

Would be good to reduce the length of the README and point to the docs.

added difficulty::easy documentation help wanted type::task labels

i can rewrite/shorten the Readme and place links to the docs as well as move stuff to the docs..

Questions:

• what has priority, the wiki or the docs page?

If its the docs page we would need to:

• Merge the open merge request. 35cd6408 to not end in a mess.
• Finish the Usage section until the flashing step (take it more or less from here.
• Adding Examples section (throw most content from the Readme into this).
• Adding Debugging section (fill from [Readme(https://gitlab.postmarketos.org/postmarketOS/pmbootstrap/-/blob/master/README.md?ref_type=heads)]
• Then after accepting the docs merge, we can shorten the Readme.
• Are we still in alpha state or do we use 3.x as the official version now therefor rename all titles from 3.0.0_alpha to pmbootstrap 3.x.

Or is the Docs section more ment to mention all flags and stuff then writing on how to use it?

mentioned in merge request !2469 (merged)

first step initiated, see !2469 (merged), could not figure out where to rename the title: 3.0.0_alpha, maybe its in the conf.py files ?

mentioned in issue #2515