docs: update the installation instructions (#317)
* docs: update the installation instructions
- update the out-of-date instructions in the manual
- add the pip-based instructions
- move the manual process from README.md into the manual: we want to
keep the README.md shorter since that's what people will see first,
and put the long, rarely needed details in the manual, where they can
be found if needed but are not in your face.
Fixes: #305
* docs: clarify that the three install methods are alternatives
Co-authored-by: Aidan Pine <aidanpine@shaw.ca>
