Documentation and node rendering

[0.5.0] - Documentation and node rendering - 2024-08-20

This version is intended to be the last release that is marked as
a beta / pre-1.0 version.

New features:

• Support for arbitrary SVG node contents, via the node_builder
  decorator
• Support for handling of custom tree node classes, via the TreeOptions
  option tree_split, as well as a pre-provided handler for nltk's
  ProbabilisticTree class
• Built-in node builder (svgling.core.subscript_node) for nodes that
  have a subscript
• A figure class HTMLSideBySide that allows mixing hybrid HTML diagrams
  with SVG diagrams

Documentation:

• Major documentation improvements, convert docs to quarto

Fixes, improvements, changes:

• Support a repeated call notation for most TreeLayout public functions
• Improvements to the draw_tree and tree2svg APIs
• Fix for constituent underlines at the exact lower edge of a figure
• Compatibility mode for rendering (some) svgling.html diagrams in quarto
• Refactor of the node construction API that goes along with the node_builder
  change
• Build/install improvements