Library description does not provide pointers to key terms

[TimurSadykov]

Current description is hard to navigate and missing some useful description. We want to improve that to help users to have a chance to self-discover actual use-case for the library

1. Short descriptions for the main concepts like Application Default Credentials. This should provide better value than enumeration of classes right now
2. Missing table of contents, impossible to navigate effectively
3. Suboptimal structure, where most important artefact description is hidden in the middle of the doc
4. CI links are outdated
5. Still says version is unstable