doc: Add radio propagation

[pradeeka7]

Summary

Add Radio Propagation tutorial for The Things Advanced + Network Management

Screenshots

Changes

• new document
• ...

Notes for Reviewers

@benolayinka Could you please provide new images for each type of propagation based on the following document? https://docs.google.com/document/d/154WxryZxFyeRNCf03HaqPHC34rLmC_jlHx3UAXNUZeA/edit?usp=sharing

This document is mainly based on: TTN Webcast | Radio Theory Crash Course - YouTube

Checklist

• [X] Scope: The referenced issue is addressed, there are no unrelated changes.
• [X] Run Locally: Verified that the docs build using make server, posted screenshots, verified external links.
• [X] Style Guidelines: Documentation obeys style guidelines in CONTRIBUTING.
• [X] Commits: Commit messages follow guidelines in CONTRIBUTING, there are no fixup commits left.

[jpmeijers]

Reflections are often allowed to get signals even in dense urban environments. - the wording in this sentence is a little strange. Maybe rephrase?

[pradeeka7]

Reflections are often allowed to get signals even in dense urban environments. - the wording in this sentence is a little strange. Maybe rephrase?

Thanks. will rephrase.

I have rephrased it as: PS- Reflections help to receive signals even in dense urban environments.

[pradeeka7]

I withdraw this pull request.