docs(bookmark): Add technical doc

[schloerke]

Fixes: #1913

[Copilot]

Pull Request Overview

This PR updates the bookmarking documentation and integrates bookmarking-related serializer functionality into input handling. Key changes include:

• Importing and exposing bookmark serializer functions via all.
• Updating input handler functions to utilize the new serializer for non-bookmarkable inputs (e.g. passwords).
• Refining comments, docstrings, and renaming internal variables to clarify bookmarking behavior.

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
shiny/input_handler.py	Integrates bookmark serializer into input handlers and updates documentation.
shiny/bookmark/init.py	Exposes new serializer functions through updated all definitions.
shiny/bookmark/_serializers.py	Adjusts serializer function parameters and formatting.
shiny/bookmark/_save_state.py	Renames the on_save callback variable to _on_save for clarity.
shiny/bookmark/_bookmark.py	Removes redundant TODO comments and improves docstring formatting.

Comments suppressed due to low confidence (2)

shiny/input_handler.py:160

• [nitpick] The use of '_' as a function name for input handlers is ambiguous and may reduce code clarity; consider using a more descriptive name for these functions.

def _(value: str, name: ResolvedId, session: Session) -> str:

shiny/input_handler.py:173

• Since the file input handler now has conditional logic for None values, please ensure that tests are added to verify the behavior when no file is provided.

    if value is None: