[FEAT] reorganize the project to match with the structure defined in #24

[cjdcordeiro]

Is your feature request related to a problem? Please describe. The goal is to prepare the project to be aligned with the ROCKs story, creating a sustainable business logic for managing the .NET ROCKs.

Describe the solution you'd like The intended structure is proposed in https://github.com/ubuntu-rocks/dotnet/issues/24. In short, we are looking at:

• main branch with a single README.md file for exploratory purposes
• branches who drive the building of ROCKs will follow a strict, per .NET version, naming convention
• each ROCK branch will consist of a Rockcraft project, with Rockcraft YAML files driving what needs to be built by the CI
• multi-base and multi-arch build are simplified into the same branch, indicated by the Rockcraft project, and dealt with by the CI/CD processes

Describe alternatives you've considered Other alternatives were considered and eventually dismissed in https://github.com/ubuntu-rocks/dotnet/issues/24.

Additional context The solution should be achieved in two stages:

1. once the org CI/CD is ready, create a build branch for testing the workflows
2. once the build branch is functional for a specific version, re-populate the main branch

[cjdcordeiro]

let's keep the main branch as "latest" for now