Problem: the README and vimdoc presets list omitted `plantuml` and
`mermaid` after both were added.
Solution: add both presets to the vimdoc table and the README features
blurb.
Problem: The vimdoc used `preview.nvim.txt` filename and
`*preview.nvim-xyz*` tags, inconsistent with other plugins.
Solution: Rename to `preview.txt`, normalize tags to `*preview-xyz*`,
add contents/install sections, and use `{field} (type)` formatting.