fix: convert tables and blocks wrapped in div containers on download

[blaipr]

Problem

The element walk in confluence_storage_to_md only renders the block elements it meets at the top level; anything else falls to the inline fallback, which keeps bare text. Confluence wraps every table saved from its editor in <div class="table-wrap">, so a page's tables downloaded as their cell texts run together (A1), and headings, lists and paragraphs inside any other <div> were flattened the same way (<div><p>a</p><p>b</p></div> → ab). Inside table cells and list items, consecutive <p> children also ran together.

Change

• A new step before the walk unwraps div, section, article, main and center, so the block elements they hold are rendered normally.
• _inline_md sets <p> / <div> children off with newlines instead of concatenating them. Table cells already turn newlines into spaces (first second); list items now indent continuation lines under the item text (- a\n b), which also fixes <br> inside an item, whose second line was previously emitted unindented.

Compatibility

Download only. Tables from the Confluence editor now download as Markdown tables; files downloaded before this change from pages containing wrapped tables or <div> content were incomplete and should be re-downloaded. Uploaded markup is unchanged.

Checklist

• ruff check src tests, ruff format --check src tests and python -m pytest tests/ pass
• Tests added for the change (tests/test_storage_to_md.py)
• Docs updated (docs/conversion.md: Block elements, Tables)
• docs/conversion.md updated because an equivalence changed
• CHANGELOG.md updated under Unreleased