Merge example page into welcome screen

[jprochazk]

What

• Part of Unify welcome and example page #4961
• Closes Welcome + Examples screen design touch-ups #3244

Changes:

• Updated design of thumbnails on example page to match https://rerun.io/examples
• Updated some spacing on welcome screen to better match design from Welcome + Examples screen design touch-ups #3244
• Remove loading spinners on example page thumbnails
• Remove tabs on welcome screen + remove the examples welcome screen card + the whole mechanism of switching between the welcome screen and example page
• Move example page contents below welcome screen
• If example page is not visible, indicate their presence with a floating See examples button, which may be clicked to scroll the examples page into view

I have not done anything to change the loading behavior, or added any extra effects as the example page loads in. There are also a few things in #4961 not done in this PR, such as updating the copy or merging the quick start examples.

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Using newly built examples: app.rerun.io
  • Using examples from latest main build: app.rerun.io
  • Using full set of examples from nightly build: app.rerun.io
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG
• If applicable, add a new check to the release checklist!

• PR Build Summary
• Docs preview
• Examples preview
• Recent benchmark results
• Wasm size tracking

[jprochazk]

First pass at updating the example page styles:

It's not exactly the same as https://rerun.io/examples, because we want to keep the size of each example, and I couldn't think of a better place to put it.

[abey79]

This is such a great improvement!

Code generally lgtm, with a few minor comments.

[jprochazk]

• Changed the Page in ExamplePage and WelcomePage to Section, and updated all references. The whole container is still called WelcomeScreen.
• Did some minor refactoring of the panel buttons in the welcome section, there was some code leftover which was previously used for switching to the example page.
• Updated the icon to 28x28 px