Bokeh: Clarify process for working on BokehJS locally

[hyles-lineata]

Is your feature request related to a problem? Please describe.
When developing locally, including changes to BokehJS, the env var BOKEH_RESOURCES needs to be set to inline for the local js to be used and changes to show up.

This env var can be seen in an example in the Developer Guide under "Next Steps," but I am proposing moving it up, explaining what it does, and making it clear that it's necessary when making JS changes.

I got bitten by this this weekend. :) I also heard it came up in the most recent sprints, but I don't know much more about that (cc @pavithraes ?).

Describe the solution you'd like
A description of the purpose and effect of BOKEH_RESOURCES=inline probably under the "Building and Installing" section of the Developer Guide (at least).

Describe alternatives you've considered
n/a

Additional context
cc @tcmetzger @bryevdv

[mattpap]

the env var BOKEH_RESOURCES needs to be set to inline

It doesn't have to, it's just one of the possible options. Alternatively one could use BOKEH_RESOURCES="server-dev" with an accompanying BOKEH_DEV=true bokeh static resources server. This makes development much easier, allows to use source maps, etc.

[mattpap]

One should also consider https://docs.bokeh.org/en/latest/docs/dev_guide/bokehjs.html for any long term "working on bokehjs", though that focuses only on bokehjs development experience, independent of the client setup (e.g. Python vs Scala, etc.), though bokehjs is still biased towards Python (e.g. defaults' tests).