Be more careful with mutated sample data in docs examples

[bryevdv]

Currently, the :bokeh-plot: sphinx directive runs all examples in a given docs build run in a single process. If the examples modify mutable data sources from bokeh.sampledata this can cause subtle and order-dependent build failures. (ref: https://github.com/bokeh/bokeh/pull/11599) There are a couple of possible ways to mitigate this:

• always return a copy from bokeh.sampledata e.g. perhaps with a module-getattr
• run docs example code in separate subprocesses

Or possibly both.