Update docs

[yasahi-hpc]

Resolves #48 This PR aims at improving the documentation. Following changes are made.

• [x] Add explanations for default arguments rather than just saying it is optional
• [x] Add a link to unmanaged view example introduced in #101
• [x] Claim that runtime errors are all given with C++ std::runtime_error (not assertion)
• [x] Add explanation about the meanings of axes which may be non-trivial for non-python users