Fix Issue 4305 - Fixed documentation of expect api

[amitbiswal007]

This PR fixes the issue 4305 https://github.com/nightwatchjs/nightwatch/issues/4305

Hi @garg3133 , Kindly review the PR.

Fix provided:

• Content of the included md files have been moved to the index md files.
• The earlier included md files have been removed as part of cleanup as these are no longer referenced across the project.