docs: document plantuml and mermaid presets

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.

flake.nix

@@ -19,9 +19,9 @@
     {
       formatter = forEachSystem (pkgs: pkgs.nixfmt-tree);
 
-      devShells = forEachSystem (pkgs:
-        let
-          devTools = [
+      devShells = forEachSystem (pkgs: {
+        default = pkgs.mkShell {
+          packages = [
             (pkgs.luajit.withPackages (
               ps: with ps; [
                 busted
@@ -32,22 +32,7 @@
             pkgs.stylua
             pkgs.selene
             pkgs.lua-language-server
-          ];
-        in
-        {
-          default = pkgs.mkShell {
-            packages = devTools;
-          };
-          presets = pkgs.mkShell {
-            packages = devTools ++ [
-              pkgs.typst
-              pkgs.texliveMedium
-              pkgs.tectonic
-              pkgs.pandoc
-              pkgs.asciidoctor
-              pkgs.quarto
             pkgs.plantuml
-              pkgs.mermaid-cli
           ];
         };
       });

lua/preview/presets.lua

@@ -302,37 +302,6 @@ M.plantuml = {
   open = true,
 }
 
----@type preview.ProviderConfig
-M.mermaid = {
-  ft = 'mermaid',
-  cmd = { 'mmdc' },
-  args = function(ctx)
-    return { '-i', ctx.file, '-o', ctx.output }
-  end,
-  output = function(ctx)
-    return (ctx.file:gsub('%.mmd$', '.svg'))
-  end,
-  error_parser = function(output)
-    local diagnostics = {}
-    for line in output:gmatch('[^\r\n]+') do
-      local lnum = line:match('^%s*Parse error on line (%d+)')
-      if lnum then
-        table.insert(diagnostics, {
-          lnum = tonumber(lnum) - 1,
-          col = 0,
-          message = line,
-          severity = vim.diagnostic.severity.ERROR,
-        })
-      end
-    end
-    return diagnostics
-  end,
-  clean = function(ctx)
-    return { 'rm', '-f', (ctx.file:gsub('%.mmd$', '.svg')) }
-  end,
-  open = true,
-}
-
 ---@type preview.ProviderConfig
 M.quarto = {
   ft = 'quarto',