docs: indent release-note continuation blocks for MkDocs

towncrier indents continuation paragraphs and nested lists by two spaces,
which Python-Markdown renders outside the list item. Indent them four
spaces and separate following items with a blank line, as ci/lint_docs.py
requires.

Assisted-by: ClaudeCode:claude-fable-5-1

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Author: d-v-b