docs: add SyncTeX section with viewer recipes (#50)

* 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.

flake.nix

@@ -49,6 +49,8 @@
               pkgs.quarto
               pkgs.plantuml
               pkgs.mermaid-cli
+              pkgs.zathura
+              pkgs.sioyek
             ];
           };
         }