docs: Add documentation for initial interval selection date ranges

[dsmedia]

This pull request adds documentation to parameters.rst to explain how to set initial date ranges for interval selections using Python's native datetime.date and datetime.datetime objects.

Changes:

• Added a new sub-subsection "Setting Initial Values" within the "Interval Selections" subsection of parameters.rst.
• Included a code example demonstrating how to use alt.selection_interval with the value parameter and Python datetime.date objects to set an initial date range.
• The new section is placed before the existing explanation of how interval selections work, maintaining the overall structure of the "Selection Types" section.

Context:

This documentation update addresses issue #3643, which requested clarification on how to use initial values in selection intervals with dates. This enhancement is made possible by the recent improvements to datetime support in Altair (PRs #3653 and #3662).

This PR specifically focuses on documenting the usage within parameters.rst. Further documentation updates related to #3643, such as adding information to the selection documentation and potentially the times_and_dates section, may be considered in separate pull requests.

Testing:

• Verified that the documentation renders correctly locally.
• Tested the code example and confirmed it works as expected.

Link to issue #3643
Link to PR #3653
Link to PR #3662

[dangotbanned]

@dsmedia big thank you for getting to this!

Flow

#3643 (comment)

You mentioned 3 potential solutions, I think some narrative docs might work well here:

• https://altair-viz.github.io/user_guide/interactions/parameters.html#selection-types

When I suggested that, I seem to have missed this pretty important part of those docs 🤦‍♂️

(emphasis mine)

Now that we have seen the basics of how we can use a selection to interact with a chart, let’s take a more systematic look at some of the types of selection parameters available in Altair.
For simplicity, we’ll use a common chart in all the following examples; a simple heat-map based on the cars dataset.

Have you tried reading from Selection Types to see how it flows together?

On my read, it seems a bit disjointed:

• cars is not used in the first example
• Neither example is a heat-map
• All of this is before the sentence that introduces that you can provide arguments

  The selection_interval() function takes additional arguments to customize behavior.

Interaction

The examples demonstrate how to set initial values - but not why you'd want to.

Your interval_selection example from (#3667) does this much better - since the interaction is mapped to an encoding channel property.

Here we have an interval but nothing changes for data in/outside it 🤔

Current screenshot

Summary

I still believe we could benefit from narrative docs on this topic - and this PR makes the first steps towards that.
Would you be able to take another stab at it with these concerns in mind?

Note

Super grateful for all the work you've been putting in over at vega_datasets 🎉

[dsmedia]

Perfectly valid feedback @dangotbanned . Here's try two, which reorders, smooths out the flow and expands a bit. (I see there was a problem with one of the checks but I believe this may be an unrelated issue?)