Bluenet documentation introduction

[martijnvandermarel]

Updates docs/README.md to provide an introduction to the many documentation pages in the docs folder, specifically aimed at developers who are new to bluenet and wish to dive deeper into it.

Contribution is twofold:

• Clusters documentation pages per function, with short descriptions on what can be found on each documentation page.
• Provides a dictionary of terms and acronyms (can be extended further) of mostly bluenet / nordic / BLE specific terms which may be unknown for new developers.

Deletions in the commits of this PR are redundant since all deleted information was already (and is still) available in the top-level README.

This PR is not intended as a complete review of the documentation of bluenet.

[vliedel]

Thanks! The documentation can always use improvements :)