[documentation] update doc for v0.5

[zyaoj]

What does this PR do? Please describe:

• Add basic doc on assets
• Bring back (updated) installation instructions
• Bring back (updated) add model tutorial + pudb tutorial
• Introduce doctest
• Update some API references

N/A

Check list:

• Was the content of this PR discussed and approved via a GitHub issue? (no need for typos or documentation improvements)
• Did you read the contributor guideline?
• Did you make sure that your PR does only one thing instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests?
• Did you verify new and existing tests pass locally with your changes?
• Did you update the CHANGELOG? (no need for typos, documentation, or minor internal changes)

[cbalioglu]

Thanks for last minute improvements to the documentation! Left a few comments, mostly nit ones.

[cbalioglu]

Nice, I think we are almost there. Left a few final comments.

[cbalioglu]

LGTM!