feat: style guide for documentation

[pkafei]

Description

Created technical writing style guide for Appsmith.

Fixes

Ensures that the documentation sounds approachable to new users and is written with a unified voice. A style guide solves the problem of "this sounds like an engineer wrote this," or "is my description of this new feature clear enough?". Finally, the style guide helps new contributors write documentation that matches the tone of existing Appsmith docs.

Type of change

Created a new guide within Appsmith's contribution directory.

@Pranay105