Improve Legend support for Layout and add documentation #1404

Job	Run time
Run pre-commit hooks	22s
UI tests on ubuntu-latest with Python 3.9	7m 28s
Pytest on 3.8, Bokeh 2, ubuntu-latest	35m 25s
Pytest on 3.8, Bokeh 3, ubuntu-latest	43m 46s
Pytest on 3.11, Bokeh 2, ubuntu-latest	38m 19s
Pytest on 3.11, Bokeh 3, ubuntu-latest	39m 17s
Pytest on 3.8, Bokeh 2, macos-latest	48m 58s
Pytest on 3.8, Bokeh 3, macos-latest	56m 33s
Pytest on 3.11, Bokeh 2, macos-latest	48m 2s
Pytest on 3.11, Bokeh 3, macos-latest	51m 35s
Pytest on 3.8, Bokeh 2, windows-latest	48m 14s
Pytest on 3.8, Bokeh 3, windows-latest	52m 30s
Pytest on 3.11, Bokeh 2, windows-latest	49m 3s
Pytest on 3.11, Bokeh 3, windows-latest	54m 53s
	9h 34m 25s

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