Docs - Overhaul EasyInvoice docs to include multiple pages, step-by-step instructions, updated screenshots, Features and supported currencies lists

[MantisClone]

Problem

1. Currently, the EasyInvoice Docs are a single long page with mostly screenshots, a few videos, and very little text.
2. EasyInvoice docs are embedded in the Request Network API page group, when it really should have it's own page group
3. EashInvoice is the entrypoint for most projects looking to integrate Request Network API and should be highlighted above the Request Network API page group.

[MantisClone]

Correction

EasyInvoice docs will be integrated with the Use Cases - not a separate section. And no detailed step-by-step instructions to use EasyInvoice (it should be self-explanatory)