No visual change, but makes the markup itself easier to read

…d/altair into when-then-user-guide

- Replaced `dict` with `when-then-otherwise`
- Replaced string expressions with `python` equivalent
- Formatting update to avoid horizontal overflow

…d/altair into when-then-user-guide

First draft version. Unlike the other commits, this has to introduce the concept - so will require more scrutiny.

vega#3544 (comment)

- Mostly correcting formatting issues.
- Removed some immediately repeated info that followed `when`.
- Added a ref to `alt.Chart`

vega#3544 (comment)

Covers all the examples that had a *methods* equivalent.
Now have 16 more to do, across 12 files

…d/altair into when-then-user-guide

All of the charts using this function were silently producing no output.
`brush` had been defined globally in an earlier directive

vega#3544 (comment)

Not sure what the title should be, but @joelostblom proposed making this a new subsection

vega#3544 (review)

https://altair-viz.github.io/gallery/interactive_bar_select_highlight.html

To be able to fully document the behavior of `when`,
I made a few organizational changes:

- Split "Conditions" and "Filter" since these are two
  different concept anyways.
- I also renamed the first heading to illustratet that
  the three big concepts are "Parameter", "Conditions", and "Filters".
- Named the main new "when" section "Understanding when()"
  and included an example of conditional branches in pseudocode early on
  and a runnable example later.
- I also added an additional subheading for "Linking across charts"
- Included mention of the use of `empty=False` inside `when` together
  with a short pseudocode example.
- Creating a new section at the end about combining parameters,
  which contains both the previous section on parameter composition,
  but also two new subsection on multiple conditions and conditional
  when branches.
- I expanded on the parameter composition examples as well.