Provide online interactive example for DTKDeclarative UI library

[ZhongYic00]

Like in https://zhuzichu520.github.io/ .QML should be easy to publish online, although Qt-wasm is very huge...
At least, a components gallery is necessary.

[BLumia]

.QML should be easy to publish online, although Qt-wasm is very huge...

dtkdeclarative is somewhat bound with DDE's QPA, not sure if it's doable. cc @18202781743

At least, a components gallery is necessary.

There is a dtk-exhibition binary served for this purpose.

[18202781743]

Now we only have some examples of documents.
Afterwards, we will continue to optimize dtk-exhibition.