/pwa-installer/ -- Clarify that the filter buttons are mockup

[mikehoffms]

Clarify that the Filter buttons are mockup only:

• new!
• PWA
• CSS
• all demos

The demo/mockup page says "You can filter the type of demo with the buttons above." That's not actually the case.

https://microsoftedge.github.io/Demos/pwa-installer/ --

138 (not supported):

140 with the two flags set:

Readme doesn't mention these buttons --
https://github.com/MicrosoftEdge/Demos/tree/main/pwa-installer/

@diekus

[mikehoffms]

The buttons didn't work before moving demo.

todo: adjust wording in .html & Readme (while the buttons aren't hooked up). Say they are mockup prototype and not hooked up yet, like other demos Readmes have a TODO section/list.
I could do this later eg in the "Move two demos" PR.

Probably have minimal instructions in demo page, just say see Readme.
Say something about intention of demo, that these are PWA; what "install" means.

@captainbrosset @diekus