docs: add worked examples for the remaining forms

[mcanouil]

The examples page showed a section, a div, a line range, a dedent and a shift. It now covers the rest, filled in place, with three more sample files under docs/_examples/.

The whole file with no fragment, a single line, the canonical shift-heading-level-by spelling rather than the shift alias the page always used, a negative shift that promotes headings, a heading pushed past level one, what happens to links inside an included file, a missing target, and a URL. The last two print their calls rather than running them, so the site's own build neither fails nor fetches anything.

A heading pushed past level one becomes a bold paragraph and loses its identifier, which the page states and its example relies on.

Two corrections to the reference page came out of the reading, and one is worth attention.

The validation table promised that every failure leaves the surrounding document intact and removes only the shortcode. A target that cannot be read or fetched stops the render instead. pandoc.mediabag.fetch raises rather than returning nothing, so the extension's if not fetched guard and the error message beneath it are never reached. Running the fetch directly under the pandoc Quarto ships confirms it. The fix belongs in the Lua, as a pcall around the fetch, and until then the page says what happens.

The dedent note said an unrecognised value is read as if the attribute had not been written, without a warning. The first half is still true and the second is not: the pinned validator coerces only true and false, so the four other spellings the extension honours are reported, and so is anything it ignores.

Verified with a full quarto render docs, no warnings and no unresolved cross-references, markdownlint-cli2 clean, all five reference anchors resolving, and no duplicate identifier on the page. Quoted messages were copied from real failed renders rather than reconstructed.