Restructure and clarify interactive docs (#2981)

* Avoid importing multiple times

* Improve wording

* Improve wording and shorten titles

* Move filtering data section

* Improve wording

* Clarify widget section

* Add search box examples

* Add legend binding section

* Clarify expression section

* Swap place of selection targets and parameter composition

* Make column selection example easier to find

* Include color picker example

* Fix link and heading

* Standardize expr and datum imports

* Clarify differences between data-driven lookups and logic driven comparisons more clearly and change search example

* Fix typo

* Restructure widget section again

* Apply suggestions from code review

Co-authored-by: Mattijn van Hoek <mattijn@gmail.com>

* Mention function under their respective headings only

* Clarify search and slider examples

* Add note about spacing

* Clarify parameter subtitles

* Mention shortcut to create legends

* Mention that a default value is required in dropdowns

* Change columns to be more consistent with intro section

* Use more idomatic approach for empty selection

* Apply suggestions from code review

Co-authored-by: Mattijn van Hoek <mattijn@gmail.com>

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Co-authored-by: Mattijn van Hoek <mattijn@gmail.com>