Update documentation for implementing a new SQL dialect adapter

[chiaradiamarcelo]

Currently the documentation for implementing a new SQL dialect adapter is not up to date, and a little bit confusing, as in https://github.com/exasol/virtual-schemas/blob/master/doc/development/developing-sql-dialect/developing_a_dialect.md

We should update it, and maybe we can use https://www.exasol.com/en/resource/an-introduction-to-exasol-virtual-schemas-the-first-part-of-our-blog-series/ and https://www.exasol.com/en/resource/how-to-build-your-own-virtual-schema-adapter-part-1/ as reference.