Documentation completion

[QuanyiLi]

What changes do you make in this PR?

• Provide a detailed explanation for MetaDrive
• Clean up legacy code including nuPlan, Argoverse, and so on.
• Slightly refactor map creation code

close #454

It is a part for #425

Checklist

• I have merged the latest main branch into current branch.
• I have run bash scripts/format.sh before merging.
• Please use "squash and merge" mode.

[github-actions]

[BOT] Great job! You have changed the docstring coverage from 28.17% to 28.19%, improving 0.017%.

[github-actions]

[BOT] Great job! You have changed the docstring coverage from 28.17% to 28.19%, improving 0.017%.

[github-actions]

[BOT] Great job! You have changed the docstring coverage from 28.17% to 28.19%, improving 0.017%.

[github-actions]

[BOT] Great job! You have changed the docstring coverage from 28.30% to 29.30%, improving 1.006%.

[github-actions]

[BOT] Great job! You have changed the docstring coverage from 28.30% to 29.32%, improving 1.018%.

[github-actions]

[BOT] Great job! You have changed the docstring coverage from 28.30% to 29.32%, improving 1.018%.

[github-actions]

[BOT] Great job! You have changed the docstring coverage from 28.30% to 29.32%, improving 1.018%.

[github-actions]

[BOT] Great job! You have changed the docstring coverage from 28.30% to 29.32%, improving 1.018%.

[github-actions]

[BOT] Great job! You have changed the docstring coverage from 28.30% to 29.31%, improving 1.008%.

[github-actions]

[BOT] Great job! You have changed the docstring coverage from 28.30% to 29.31%, improving 1.008%.