Add docs for ContentPage, NavigationPage, TabbedPage, DrawerPage, and CommandBar

[jsuarezruiz]

Changes:

• Adds reference documentation for the five native Avalonia Page controls: ContentPage, NavigationPage, TabbedPage, DrawerPage, CarouselPage and CommandBar (with AppBarButton, AppBarToggleButton, AppBarSeparator)
• Each doc covers: properties table, enum value tables, events, gestures/keyboard behaviour, and examples in both XAML and C#
• Screenshots added for all five controls under static/img/reference/controls/

[luke-whos-here]

Left some comments and suggestions on the relevant files. I also have two overall remarks for the PR as a whole:

1. With the release of the v12 docs, the .md files are now under the /controls/navigation path, and the images are now under the /static/img/controls path. I would recommend moving these to the new file structure before merging.
2. Some of these pages feel like they would benefit from cross-linking where other controls are mentioned, e.g., TabbedPage has an extensive example covering interaction with NavigationPage. Kind of discretionary, so feel free to add some or not as you think appropriate.

[luke-whos-here]

Suggested change

	- Horizontal tabs (Top, Bottom): left and right arrow keys switch tabs. RTL layouts reverse the direction.
	- Horizontal tabs: left and right arrow keys switch tabs. RTL layouts reverse the direction.

[luke-whos-here]

Not sure if it really needs a detailed explanation, but this does make me wonder how preference is decided if there are two tabs that are both the nearest. (e.g., one on the left and one on the right)

[luke-whos-here]

Suggested change

	Each `NavigationPage` maintains its own back stack independently. Switching tabs does not reset the stack.
	Each `NavigationPage` maintains its own stack independently. Switching tabs does not reset the stack.

[luke-whos-here]

Left some comments and suggestions on the relevant files. I also have two overall remarks for the PR as a whole:

1. With the release of the v12 docs, the .md files are now under the /controls/navigation path, and the images are now under the /static/img/controls path. I would recommend moving these to the new file structure before merging.
2. Some of these pages feel like they would benefit from cross-linking where other controls are mentioned, e.g., TabbedPage has an extensive example covering interaction with NavigationPage. Kind of discretionary, so feel free to add some or not as you think appropriate.

[jsuarezruiz]

Thanks for the thorough review! All inline suggestions have been applied. Regarding the two overall remarks:

1. File paths: Moved files to /controls/navigation and images to /static/img/controls.
2. Cross-linking: Added cross-references where controls are mentioned in examples (e.g., TabbedPage referencing NavigationPage, DrawerPage referencing NavigationPage integration).