docs: use RAUC image in the Yocto build instructions

[reubenmiller]

Proposed changes

Use the tedge-rauc yocto project to demonstrate firmware (OS) updates instead of mender.

Additional changes

• Remove unused pylint3 dependency (not required in >= Kirkstone)
• Add git-lfs installation instructions as it is still required when installing thin-edge.io from source in Yocto

Types of changes

• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Improvement (general improvements like code refactoring that doesn't explicitly fix a bug or add any new functionality)
• [x] Documentation Update (if none of the other choices apply)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Paste Link to the issue

• https://github.com/thin-edge/meta-tedge-project/pull/19

Checklist

• [x] I have read the CONTRIBUTING doc
• [x] I have signed the CLA (in all commits with git commit -s)
• [ ] I ran cargo fmt as mentioned in CODING_GUIDELINES
• [ ] I used cargo clippy as mentioned in CODING_GUIDELINES
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation (if appropriate)

Further comments

[github-actions[bot]]

Robot Results

:white_check_mark: Passed	:x: Failed	:next_track_button: Skipped	Total	Pass %	:stopwatch: Duration
432	0	3	432	100	0s

[reubenmiller]

@Ruadhri17 I just pushed one more minor change regarding the "package manager" option. Can you please review this commit 446a3fe1c544f7d3f17066b33ab3ec81f5d6c20d?