Lack of usable BLE documentation

### Description of defect
All the pages & repos that are intended to explain how to program BLE functionalities are full of non-sense bloat and are cross-linking to eachother without actually onboarding Mbed users how to use BLE.

#### Target(s) affected by this defect ?

all

#### Toolchain(s) (name and version) displaying this defect ?

all

#### What version of Mbed-os are you using (tag or sha) ?

6.8.0

#### What version(s) of tools are you using. List all that apply (E.g. mbed-cli)

Not relevant

#### How is this defect reproduced ?

Read through all the BLE relevant pages of the official Mbed site, check out this repo, forget anything that BLE engineers already know and face the reality: there is no useful info through all those pages. And guess what: even the recent examples will not compile as those depend on pretty printer. Even if that is not mentioned anywhere. Got a feeling as the whole BLE stack was not an organic part of Mbed OS just someone would have hacked that into the project and even then was it meant to function with mbed CLI only.

Review the BLE-relevant pages on the official Mbed site and the examples in this repository, starting with the recent examples and their undocumented pretty printer dependency. Identify the onboarding gaps and broken or misleading links, then document concrete BLE workflows that compile with the supported tooling.

Written by the indexing model from the issue text.