Disable Documentation site publish if it's a forked repo

[yitsushi]

What this PR does / why we need it:

It's annoying, every time I update my fork, I get a notification about the failed GitHub Action. It tries to use a secret with SSH key, but I don't have one on my fork.

Which issue(s) this PR fixes:
Fixes #250

Special notes for your reviewer:

Checklist:

• squashed commits into logical changes

[codecov-commenter]

Codecov Report

Merging #251 (9d9466d) into main (fe9bb9f) will decrease coverage by 15.61%.
The diff coverage is 41.78%.

@@             Coverage Diff             @@
##             main     #251       +/-   ##
===========================================
- Coverage   55.81%   40.20%   -15.62%     
===========================================
  Files          44       46        +2     
  Lines        2012     2169      +157     
===========================================
- Hits         1123      872      -251     
- Misses        781     1237      +456     
+ Partials      108       60       -48     

Impacted Files	Coverage Δ
core/application/commands.go	71.66% <ø> (+2.70%)	⬆️
core/application/reconcile.go	0.00% <ø> (ø)
core/models/vmid.go	90.24% <ø> (ø)
core/models/volumes.go	0.00% <ø> (ø)
core/steps/microvm/delete.go	100.00% <ø> (ø)
core/steps/runtime/dir_create.go	73.07% <ø> (ø)
core/steps/runtime/dir_delete.go	81.81% <ø> (ø)
core/steps/runtime/initrd_mount.go	100.00% <ø> (ø)
core/steps/runtime/kernel_mount.go	100.00% <ø> (ø)
infrastructure/containerd/content.go	0.00% <0.00%> (-100.00%)	⬇️
... and 36 more

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