Move ubuntu-image.rst to Ubuntu Install Guide

[rkratky]

This PR removes the ubuntu-image.rst file (the former man page) because it was moved to be published as a reference page in the Ubuntu Installation Guide.

See canonical/subiquity#1929 for details (do not merge this PR until the linked one is merged -- only then the new link will become available).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.55%. Comparing base (1313d05) to head (63f75d8).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #193      +/-   ##
==========================================
- Coverage   87.33%   77.55%   -9.78%     
==========================================
  Files          15       15              
  Lines        3915     3921       +6     
==========================================
- Hits         3419     3041     -378     
- Misses        436      814     +378     
- Partials       60       66       +6     

Flag	Coverage Δ
unittests	77.55% <ø> (-9.78%)	⬇️

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[upils]

LGTM.

Do not worry about the dependencies vuln check failing, this is unrelated and will be fixed in another PR.

[upils]

I think you should also remove this check https://github.com/canonical/ubuntu-image/blob/main/.github/workflows/documentation.yml#L51 and the next one, that will not make sense anymore.

[rkratky]

I think you should also remove this check https://github.com/canonical/ubuntu-image/blob/main/.github/workflows/documentation.yml#L51 and the next one, that will not make sense anymore.

Good point.

I have fixed the link and removed the two checks.