Problem: the `markdown` and `github` presets now default to `--mathml`
but users may want KaTeX or MathJax rendering instead, and the
incompatibility with `--embed-resources` is non-obvious.
Solution: add a `preview-math` section to the presets docs explaining
the default, why `--katex`/`--mathjax` require dropping
`--embed-resources`, and a concrete recipe for KaTeX with `github`.
* docs: add SyncTeX section with viewer recipes
Problem: SyncTeX setup for forward/inverse search was undocumented,
forcing users to figure out viewer-specific CLI flags on their own.
Solution: Add `preview-synctex` vimdoc section with shared setup and
per-viewer recipes for Zathura, Sioyek, and Okular. Add FAQ entry
in README pointing to the new section.
* build: add `zathura` and `sioyek` to nix dev shell
* docs: fix Okular inverse search instructions
Problem: Okular settings path was incomplete and didn't mention the
trigger keybinding.
Solution: Update to full path (Settings -> Configure Okular -> Editor)
and note that Shift+click triggers inverse search.
* 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.
