Document copying assets

[nhoening]

We should document briefly:

• Why copying assets can save time and what examples come with FlexMeasures. Mention it on this page as well.
• copying any asset on the page for assets, with a screenshots of the button
• The new asset search choice for on the asset creation page
• For plugin authors, we should add a section to mention that plugins can make sure on startup that custom template assets exist, so users could create new accounts & sites quickly. Some example code would of course be nice - I believe code in the plugins's __init__.py would be the place to call such a function.

[nhoening]

First work already done in #2195 itself. More in #2268.