docs: add migration guide page

[steff456]

fixes #658

This PR,

• [x] Adds a new migration page
• [x] Adds the new page under the Other heading at the top-level structure

The idea is to start growing the migration page with more examples from NumPy to the Array API Standard. @asmeurer I don't know if you have other examples in mind to add them in this PR.

[asmeurer]

This looks good. My idea was indeed to have a table, like a Rosetta stone (e.g., like https://github.com/sympy/sympy/wiki/Git-hg-rosetta-stone).

I would add two notes to the text at the top:

• Some functions aren't easy to translate. In that case, if they are important, people should check if they are discussed already at https://github.com/data-apis/array-api/issues and request them if not. Also point out that people may have to use library-specific code paths for certain things that aren't in the standard yet.

• Note that people should feel free to contribute to the page.

I can see about adding a few things myself once this is merged.