Update ReadMe files after monorepo restructuring

Is your feature request related to a problem? Please describe.

After the folder restructuring, the ReadMe files need to be kept up to date as well. At the moment, the top level ReadMe also reflects the developer version instead of the marketplace version.

Describe the solution you’d like

Perhaps the new structure should be as follows:

.
│-- README.md (Top level ReadMe featured in VScode marketplace)
│
└───docs
│   │-- Developer ReadMe (Install, Build, and Test the Extension)
│   │-- README-Extending.md
│   │-- README-Security.md
│   │-- README-Theia.md
│
└───packages
│   └───eslint-plugin-zowe-explorer
│   │   │-- ZE eslint plugin ReadMe
│   │
│   └───zowe-explorer-api
│   │   │-- ZE API ReadMe
│   │
│   └───zowe-explorer-ftp-extension
│   │   │-- ZE FTP extension ReadMe
│   │
│   └───zowe-explorer
│       │-- ZE Core ReadMe

Additional context

The references to the old file structure have to be updated, too (e.g. ./resources/testProfileData.example.ts).

[jellypuno]

I can do the contributions to awesome-zowe

[crawr]

A new Doc Updates Epic was opened to track all the related issues. This issue will focus on the top level readme update