Documentation Improvements (Tutorial)

[bassoy]

@shikharvashistha formatted the current documentation using AsciiDoc during GSoC21. It is merged into the develop branch, see pull request #132 which provides a nice formatting also in Github, see https://github.com/boostorg/ublas/blob/develop/doc/ublas.adoc.

We prepared a future topic table in BoostGSoC21#53 (comment). We suggest to first prepare a tutorial and then to improve the reference in the style of https://en.cppreference.com/w/. The corresponding project is described in https://github.com/boostorg/ublas/projects/5. Any further suggestions are welcome.