[DOCS] HelloThere. Add info about the core nature of TW as a web-app

[pmario]

Adds the following info to HelloThere

* TiddlyWiki stores its data and runtime in a single HTML file, requiring no installs, no external dependencies, nothing but a web browser

Also see: https://talk.tiddlywiki.org/t/public-perseption-of-tw-from-the-first-few-tiddlers/11853/2

[github-actions]

Confirmed: pmario has already signed the Contributor License Agreement (see contributing.md)

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[mateuszwilczek]

Small suggestion: I feel like the word runtime might be a bit too unclear for less techy users, especially that its the third sentence on the entire page. Perhaps something like

* TiddlyWiki stores all its data and code in a single HTML file, requiring no installs or external dependencies, just a web browser

would be better?

[Jermolene]

Thanks @pmario I think those are good improvements, and I'd welcome further work on this.