Add documentation for the web UI

[rezib]

The documentation should be updated for the web UI:

• The feature should be mentioned in getting started and overview pages with some nice screenshots
• A new dedicated page should explain:
  • How to install and launch the UI application
  • Explain briefly to end users how to navigate in the pages with features available (search, filters, etc)

[florianLSP]

• In Quickstart:
  • In the section installation From Sources:
    • Add paragraph to explain how to install Web-UI
  • In Explore content:
    • Add a new section Web-UI under REST-API to explain that UI is included in racksdb-web when you run the command racksdb-web --with-ui
    • Add ‘more links’ section with the like of the page Web-UI
• In Getting Started:
  • Display an image with 3 screenshots of the UI
• In Overview:
  • In Interfaces:
    • Add to the schema ‘Web-ui’
    • Add a text to explain that it’s now possible to use the UI
    • Add a link to the page ‘Web-UI’
    • Display an image with 3 screenshots of the UI
• Create Web-UI page that list all the features of the UI:
• In README:
  • Remove beta mention
  • Display an image with 3 screenshots of the UI
  • Edit the link to direct it to the UI doc
• In CLI manpages racksdb-web:
  • Add the possibility to access to the Web-UI by running the command racksdb-web --with-ui

[rezib]

* In **CLI manpages racksdb-web**:
  
  * Add the possibility to access to the Web-UI by running the command `racksdb-web --with-ui`

Note that it's already done in 53f1cba.