Docs need some tidying up

[elliotgunton]

A few things are out of date/incorrect on the docs:

• Script basics "explicit" example passes source=echo in a Task - this is not possible
• Script annotations is quite confusing to read. Examples are overly complex/wordy, some being incorrect. We can also show that name can be inferred when Fix multiple bugs in Hera Input and Output classes #1193 is merged
• expr guide's first link is dead, also some formatting can be tidied up
• README is duplicated due to mascot image link, but we can fix this. Also note the PyPI page has a broken image