Unify documentation websites

[andreadm84]

There are currently 3 different documentation websites (with 3 different logos!):

• [x] https://2.docs.brightway.dev/

• [x] https://brightway.dev/

• [x] https://brightway-docs.readthedocs.io/

[andreadm84]

All documentation will be soon moved to:

User-oriented (Jupyter Book): https://documentation.brightway.dev/ Developer-oriented (API): https://brightway.readthedocs.io/