Rejuvenate visuals to match current WinUI gallery

[chrisglein]

When React Native Gallery was created, it was before the WinUI gallery had experienced a significant visual rejuvenation. We should be able to show the same class of visuals with a bit of effort. Here are the high impact items I can identify.

Splash image and set of useful links

Many of these are just as relevant within RN Gallery

• Getting started (can be link to https://aka.ms/reactnative or specifically the getting started subpage)
• Windows design
• Github
• Template Studio (are there still RNW templates in there?)
• Code samples (link to our samples repo)
• Partner Center

Recently added/updated

Design guidance

Control group subpages (with full color icons in page view)

Contrast with RN Gallery which has groups only on the main page, and the icons are monochromatic:

Work in progress snack to get started: https://snack.expo.dev/4NjQuP5ias-bJa04RLYIk?platform=web https://snack.expo.dev/wvKq-kD7PqlROjiiiswqx?platform=web

## Tasks
- [x] Add splash image to home page
- [x] Add list container for useful links
- [x] Add link to Getting started (https://aka.ms/reactnative)
- [x] Windows design (https://learn.microsoft.com/en-us/windows/apps/design/_)
- [x] Github (https://github.com/microsoft/react-native-windows/)
- [ ] Template Studio (https://marketplace.visualstudio.com/items?itemName=TemplateStudio.TemplateStudioForUWP)
- [x] Code samples (https://github.com/microsoft/react-native-windows-samples/)
- [x] Partner Center (https://developer.microsoft.com/en-us/windows/)
- [ ] https://github.com/microsoft/react-native-gallery/issues/433
- [ ] https://github.com/microsoft/react-native-gallery/issues/434
- [x] Replace monochromatic icons with full color icons (can take from https://github.com/microsoft/WinUI-Gallery/tree/main/WinUIGallery/Assets/ControlImages)
- [ ] https://github.com/microsoft/react-native-gallery/issues/435
- [x] Reshuffle groups to better align with WinUI gallery's grouping as appropriate (e.g. add Collections and move FlatList and friends into that)
- [ ] https://github.com/microsoft/react-native-gallery/issues/437
- [x] Code examples have a copy button
- [ ] https://github.com/microsoft/react-native-gallery/issues/216
- [x] Add indicator for new/updated samples
- [ ] https://github.com/microsoft/react-native-gallery/issues/436
- [x] Update readme image https://github.com/microsoft/react-native-gallery/blob/main/README.md

[chrisglein]

In checking dark mode with the rejuvenated home screen, I've also noticed some gaps in the existing code example implementation:

Before

After

Note added copy buttons.

[chrisglein]

As long as we're adding clipboard as a dependency, we should have a clipboard example:

Compare to WinUI gallery's similar clipboard sample:

[chrisglein]

And since linear gradient was needed for the home page, can create a sample for it too:

[chrisglein]

With all merged (and a couple active) PRs:

Light mode https://github.com/microsoft/react-native-gallery/assets/26607885/0b2b1a1e-5b67-491d-a0c5-dd025e16ea8f

Dark mode https://github.com/microsoft/react-native-gallery/assets/26607885/65e8ff34-a319-48b0-a3ec-75d186cf20df

[chrisglein]

With all the stuff merged so far: