Document common layout patterns

[marcushellberg]

We get lots of questions about layout usage from community members. It could be helpful if we provided some further documentation on some common patterns:

• How to combine horizontal and vertical layouts to build a more complex layout (for instance, a header, sidebar, main layout)
• How to align a single item in a layout (for instance, the last button in a horizontal layout is aligned to the end of the layout)
• How to properly size nested layouts and components

[OlliTietavainenVaadin]

Very concrete example: "Why is my Grid not showing even if it's setSizeFull(true)? There's just a line." I must have seen this question dozens of times by now.

[jouni]

@OlliTietavainenVaadin: we are considering of alleviating some of this pain by having a default min-height for Grid, so that at least won't shrink to just one line, but will always show at least one row.