docs: align toolchain and code style with C++20 library

[Bartok9]

Summary

• Document that the MAVSDK core library is C++20 (cpp/src/CMakeLists.txt CMAKE_CXX_STANDARD 20, examples already on 20, repo description “C++ 20”).
• Update the sample CMakeLists.txt in the toolchain guide to recommend CMAKE_CXX_STANDARD 20.
• Soft-update code-style guidance so new contributions/examples prefer C++20 while still welcoming clear C++17/STL usage.

Why

Docs still said the core was “written in C++17”, which confuses users trying to match the tree.

Non-overlap

Does not touch the Markdown fixes in open #2922 (that PR covers connections fence, quickstart fence, build paths, etc.).

Test plan

• Docs-only; no C++/CMake library flag changes
• Spot-check relative statements against cpp/src/CMakeLists.txt and example CMakeLists

[Bartok9]

Thanks @julianoes — good call, that second sentence was muddled. Applied your suggestion (simplified to the single C++20 line) and pushed. Let me know if you'd prefer to drop the STL note entirely.