update link to child selenium grid page when available

[vdelendik]

in #522 updated link name to Zebrunner Selenium Grid but as we don't have yet child page keep reference to the zebrunner.com.
as only we create dedicated grid page we can update reference in doc as well on overview page: https://zebrunner.github.io/community-edition/

[vdelendik]

1. renamed Zebunner Egnine to Zebrunner Selenium Grid and provided link onto https://zebrunner.com/selenium-grid.
2. renamed MCloud sub-component to Zebrunner Device Farm.
3. renamed Zebrunner Reporting to Zebrunner Testing Platform (Community Edition) and provided direct link to https://zebrunner.com/testing-platform
   Before releasing changes might be discovered here:
   https://github.com/zebrunner/community-edition/blob/develop/docs/index.md

[ghost]

verified