"path-only" for sideNaviStyle option to be validated correctly and documented.

[muraaano]

Describe the problem to be solved
Since #1805, sideNavStyle was added.
I tried to configure sideNavStyle to "path-only" then it was affected to the view but below warning were shown.

[1] redocly.yaml:8:19 at #/theme/openapi/sideNavStyle

`sideNavStyle` can be one of the following only: "summary-only", "path-first", "id-only".

 6 | hideHostname: true
 7 | pathInMiddlePanel: true
 8 | sideNavStyle: path-only
 9 |
10 | theme:

Warning was generated by the configuration spec rule.

Also, sideNavStyle is not documented.

Describe the solution you'd like

1. On generation, sideNavStyle warning for "path-only" not to be shown.
2. sideNavStyle to be documented

[tatomyr]

@muraaano what Redocly CLI version you're using?