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