Documentation outdated for 3 years

**Description**  

https://docs.guzzlephp.org is outdated. Last update was 3 years ago.

**How to reproduce**  

Visit documentation or look at RTD project site.
https://app.readthedocs.org/projects/guzzle/builds/?version__slug=&state=succeeded&version__type=

**Possible Solution**  

Add missing config. Add status badge.

**Additional context**  
Nope
