Simplify Selenium / webdriver installation and requirements

[tcmetzger]

Selenium and a web driver are used mainly in two places in Bokeh:

• For the tests
• for using the export functions (export_png, export_svg, and export_svgs)

Getting Selenium and a web driver set up and configured can be complicated and is a hurdle for new users (wanting to use the export function) and new contributors (wanting to run tests locally). See #11449 for more details.

As discussed with @mattpap and @bryevdv there are a few things to consider:

• We could make conda install selenium firefox geckodriver -c conda-forge the recommended method to use the export functions (in the user guide, https://docs.bokeh.org/en/latest/docs/user_guide/export.html). We would still explain other options in the docs as well, but give a recommendation for the (currently) easiest setup.
• @mattpap suggested we could use the Chrome/Chromium API directly for testing. This would be in addition to using selenium/webdrivers, so that people could use either Chrome directly or use Selenium with a web driver for Chrome/Firefox/etc. This would make running tests locally a lot easier (since Chrome is required for the visual tests anyways). A downside would be that the examples tests still require Selenium - in the docs, we would therefore generally recommend to not run example tests locally but have them run in the CI. If we decide to use Chrome directly, we would still explain the other options as well (in the testing section of the dev docs).
• @mattpap do I remember correctly that using Chrome/Firefox directly through their APIs would also be an option for export_png, export_svg, and export_svgs?
• As an alternative, @bryevdv suggested looking into https://github.com/pyppeteer/pyppeteer