docs: Add doc on querying/mutating data

[Pathan-Amaankhan]

What

• This PR adds a doc describing how global styles and queried template data can be mutated/updated.

Related Issue(s):

• https://github.com/rtCamp/headless/issues/314

Screenshots

Checklist

• I have read the Contribution Guidelines.
• My code is tested to the best of my abilities.
• My code passes all lints (ESLint, tsc, prettier etc.).
• My code has detailed inline documentation.
• I have added unit tests to verify the code works as intended.
• I have updated the project documentation accordingly.

[justlevine]

Not entirely sure I understand this doc. Either its the wrong focus or it's missing things.

My assumption is that we should at minimum see a snippet of graphql somewhere, ideally the use of @apollo/client or even our own @snapwp/query somewhere.

Not necessary for 0.0.1, so there's time to come at this with fresh eyes.

[SH4LIN]

We should add a example code snippet which actually overrides the functions. Along with the data that is expected from those functions

[Pathan-Amaankhan]

• This PR is migrated to docs: backfill next readme with getGlobalStyles() and getTemplateData() #46