[Rules concepts] Add triggers & conditions

[florian-h05]

@rkoshak
Unfortunately I have bad weather today on my vacation trip, so this is the next microstep in reworking the rule documentation.

This PR should complete part 1b and 1c of openhab#1855.

[rkoshak]

Over all it looks really good. Thanks for submitting. Most of these comments are nit picking but I think they are important.

[florian-h05]

@rkoshak
Ready for re-review.
Out of interest: Do you get a notification when I re-request a review?

[rkoshak]

Looks great! I've just a few typos and word order type comments and we are good to merge. Thanks again for working on this. I keep trying to find an hour where I can sit in front of a computer to type something out but it's not worked out yet.

I do get an alert through the Github app on my phone when it's a PR or issue I'm subscribed to or am mentioned in a comment. I also check the list of issues and PRs in my github notifications every couple of days. But I like to dedicate a contiguous time to do reviews like this which can delay my response.

[florian-h05]

Thanks again for working on this. I keep trying to find an hour where I can sit in front of a computer to type something out but it's not worked out yet.

You are welcome. I know the problem with the time, luckily I have school holidays and some time to work on this.

[florian-h05]

Can you please update the issue and tick the checkboxes for 1b and 1c?

[rkoshak]

Yes, already have it open, just got distracted for a bit.