Splitter(Docs): Add splitter topic

[VMihalkov]

Closes #1842

Pending before this can be finalized:

1. A sample with vertical splitter bar and 2 panes, e.g. a design surface and its corresponding code view
2. A sample with more complex layout including horizonal, vertical and nested splitters

[StefanIvanov]

I made a commit with a few minor suggestions, overall the topic structure looks excellent, are we going to add styling section (I know there is not much to be styled) and an advanced section with more elaborate sample e.g. a UI/code view for example? @simeonoff @ChronosSF

[simeonoff]

There's no styling section in this topic.

As for the samples. We could use 3 samples. All 3 samples can simply be splitting text paragraphs. The first sample could be a very simple horizontal splitter (as you have it described). The second would be a representation of the more complex splitter mentioned in the topic, again, splitting paragraphs of text. The third sample should showcase the custom theme and be a part of the missing styling section. If you have a working branch with samples, please push it, I can handle the rest.

[VMihalkov]

There's no styling section in this topic.

As for the samples. We could use 3 samples. All 3 samples can simply be splitting text paragraphs. The first sample could be a very simple horizontal splitter (as you have it described). The second would be a representation of the more complex splitter mentioned in the topic, again, splitting paragraphs of text. The third sample should showcase the custom theme and be a part of the missing styling section. If you have a working branch with samples, please push it, I can handle the rest.

@simeonoff the 3 samples have been addded in branch vmihalkov/splitter-samples