fix(gen): alias /doc/user/helptag.html to /doc/user/helptag/ #38156

Problem:
helptag.html is used to search for the online help documentation of a
specified tag. The previous URL was `/doc/user/helptag.html` but that
switched to `/doc/user/helptag/` in
https://github.com/neovim/neovim.github.io/pull/437. The alias of the
.html page was added to all other doc pages but forgotten for the
helptag.html page

Solution:
Add the alias to the helptag.html page too

src/gen/gen_help_html.lua

@@ -956,7 +956,8 @@ end
 local function gen_helptag_html(fname)
   local frontmatter = vim.json.encode({
     title = 'Helptag redirect',
-    layout = 'helptag', -- Hugo-specific
+    layout = 'helptag',
+    aliases = { vim.fs.basename(fname) },
   }, { indent = '  ', sort_keys = true })
   tofile(fname, frontmatter)
 end