Added new endpoints to flask example

[Luishfs]

Motivation

I've had some difficulty to understand the latter flask/django python examples and how DIDKit works. By adding the new endpoints along with the updated README.md, i believe new users can understand better not only how to use SpruceID DIDKit, but also how DID concepts works.

Description

This PR adds 3 new endpoints to the Flask API DIDKit example, along with a step-by-step on how to generate/validate both VCs and VPs.

New example endpoints

The first page leading after accessing the defined Flask URL now allows users to interact with 4 main components from DIDKit:

1. Issue Verifiable Credential
2. Verify Verifiable Credential
3. Issue Verifiable Presentation
4. Verify Verifiable Presentation

All cited components lead to a exclusive URL where users can interact via browser to execute the specified task using DIDKit python.

New Pages

First Page (/)

Issue VC

Verify VC

Issue VP

Verify VP