[NEW PAGE] introduction to fractional gates

[javabster]

When the device basis gates get updated to include rx and rzz gates we will need a new page in the build section to show how to use this feature to the best of it's ability.

@nkanazawa1989 has a nice notebook that could serve as the foundation for this content

Some things to include:

• what do we mean by the term "fractional gates"
• how do they differ from regular parameterised gates