Update warning for Sitemaps

[mueller-ma]

Currently there's a warning in the docs that suggests to use Main UI instead of Sitemaps for new users:

Keep in mind that the main UI is not currently able to display them. If you are a new user, it's probably a good idea to start customizing your Overview page first.

On https://www.openhab.org/docs/ui/ it says about Sitemaps: "As they have less customization options than the other types of pages, [...] they are also arguably easier to learn." So I wouldn't say new users must start with configuring the overview page, but instead read a comparison between the different UIs. Or at least suggest to read a comparison.

@rkoshak Did such a comparison in the forum that could be taken as a starting point for the docs: https://community.openhab.org/t/newbie-discussion-which-is-the-best-ui-for-mobile/119048/5

[rkoshak]

Just note that that comparison might be a little out of date. I'm also not sure there's enough information there for a beginner to make a good decision or even if a beginner can make a good decision. More probably needs to be added to the table than what's there now.