[nrfconnect] Extended DFU documentation about using mcumgr tool.

[kkasperczyk-no]

Problem

Examples documentation about performing Device Firmware Upgrade over BLE describes only case using Nordic mobile app. It would be beneficial to have also documentation that shows how to perform firmware upgrade from PC command line tool.

Change overview

• Moved DFU description from examples to separate guide readme.
• Extended documentation by case using mcumgr command line tool to perform DFU.
• Removed unnecessary part of the code allowing to advertise over BLE with non conn type that was added in very early verison
  of BLE Manager implementation.

Testing

Change introduces mainly documentation, so it was verified manually that introduced steps work properly. Moreover it was verified that changed BLE configs doesn't have negative impact and after introducing them following things work properly:

• Performing DFU over BLE with smartphone running Nordic's nRF Connect for Mobile application with Thread on accessory disabled and enabled.
• Performing DFU over BLE with mcumgr command line tool on Linux with Thread on accessory disabled and enabled.
• Device commissioning over BLE using Python CHIP controller.

[github-actions]

Size increase report for "nrfconnect-example-build" from c0ac96c

File	Section	File	VM
chip-lock.elf	text	-32	-32
chip-shell.elf	text	-32	-32

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-lock.elf and ./pull_artifact/chip-lock.elf:

sections,vmsize,filesize
.debug_loc,0,-24
text,-32,-32
.debug_info,0,-48
.debug_line,0,-48

Comparing ./master_artifact/chip-shell.elf and ./pull_artifact/chip-shell.elf:

sections,vmsize,filesize
.debug_loc,0,-24
text,-32,-32
.debug_info,0,-48
.debug_line,0,-48