docs: refactor readme.

[janfb]

• refactor first paragraphs
• add new methods
• add aAI to credits.
• fix all links in code, tutorials and FAQ because we changed "tutorial <- tutorials"

fixes #1214

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.16%. Comparing base (b46c7c5) to head (bdd535c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1235      +/-   ##
==========================================
- Coverage   86.05%   78.16%   -7.90%     
==========================================
  Files         117      117              
  Lines        8672     8679       +7     
==========================================
- Hits         7463     6784     -679     
- Misses       1209     1895     +686     

Flag	Coverage Δ
unittests	78.16% <ø> (-7.90%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
sbi/inference/trainers/base.py	86.91% <ø> (ø)

... and 28 files with indirect coverage changes

[michaeldeistler]

Thanks a ton! Left some comments

[michaeldeistler]

@janfb Could you take care of the SNPE->NPE rename in the readme?

[michaeldeistler]

Good to go!