[To Add] Index File for navigation

Add [Index file](https://github.com/BoostGSoC21/ublas/blob/features/gsoc21/shikhar/doc/ublas.adoc) to navigate the documentation

Find compiled html file [here](https://github.com/shikharvashistha/test-b2/blob/master/src/bin/bin/asciidoctor-backend-html5/ublas.html)

- How It looks

![one](https://user-images.githubusercontent.com/51105234/128625955-e808407d-4489-470b-86e9-06932791c18f.png)
