* feat: add `extra_args` provider field
Problem: Overriding a single flag (e.g. `-outdir=build`) required
redefining the entire `args` function, duplicating all preset defaults.
Solution: Add `extra_args` field that appends to the resolved `args`
after evaluation. Accepts a static table or a context function.
* docs: document `extra_args` provider field
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.