Update and Unify the docs for populating data on the dev server and accessing the routes

Problem

Currently, our docs for data population and accessing a certain route are scattered across 2-3 docs, which makes it difficult for new contributors to refer to these docs. A few of these scattered docs are:

• https://github.com/oppia/oppia/wiki/How-to-access-Oppia-webpages
• https://github.com/oppia/oppia/wiki/Populating-data-on-local-server
• https://github.com/oppia/oppia/wiki/LaCE-onboarding-guide

Solution

Create a single doc that can be used by the contributors to understand the data population on the dev server and access all the routes present in the Oppia web.

To find all the routes that are present currently in the Oppia Web, check out this file: https://github.com/oppia/oppia/blob/develop/core/templates/pages/oppia-root/routing/app.routing.module.ts