Update docs for custom styles to use sidebar class

[fty4]

Description

The UI of Argo CD changed where no .nav-bar class is present anymore.
In the new release the class changed to .sidebar.

The documentation still has the old implementation referenced to modify the menu color which can be changed optionally.

The documentation here should be fixed and changed to reference the new sidebar class.

The old color suggestions might not be the best (linear-gradient does not match with Argo CD style anymore?).
I would suggest to change it as in this commit here:

fty4@e853963

Do you agree? Or how should this be changed?

---

original Argo CD style

Only class change

.sidebar {
  background: linear-gradient(to bottom, #999, #777, #333, #222, #111);
}

class and color change

.sidebar {
  background-color: #444;
}

[saumeya]

Thanks for opening the issue @fty4 , Would you like to contribute these changes to the doc?

[fty4]

@saumeya I've created a PR (#11733)