Add an opt-in check for a single document element

[MaciejZet]

Summary

One slice of #848: a document has exactly one element.

Config::check_document stays off, so <a/><b/> and other fragments still parse. With the flag on, the reader returns Error::IllFormed for:

• a second top-level element
• text, CDATA, or a reference outside that element (whitespace around it is fine)
• end of input with no element, including an XML declaration on its own

Comments and processing instructions around the element stay allowed. enable_all_checks still covers only the comment and end-name checks, so existing fragment callers are unchanged.

The rest of #848 is still open. This does not try to close it.

Test plan

• cargo test --all-features --lib --tests
• doctest on Config::check_document
• cargo fmt -- --check

[Mingun]

I prefer that such check would be performed by the quick-xml consumers.