Adopt the shared Pandoc template and drop the local fork

[t4k]

docs.yml passes template: pandoc/page.tmpl, a local copy that predates the
shared theme. Compared with caltechlibrary/workflows/pandoc/page.tmpl it is
missing:

• <meta charset="utf-8">
• <meta name="viewport" content="width=device-width, initial-scale=1">
• a parameterized title — every page here renders <title>CL-web-components</title>
• the $if(search)$ and $if(repo-url)$ conditionals
• $site-base$, $for(extra-css)$ and $for(extra-js)$

This repository also still holds pandoc/css/code-blocks.css and
src/copyToClipboard.js. The shared theme now owns both and ships them into
every site it builds, so these are duplicates once the template switches.

The blocker

The local template hand-lists nav items the shared one does not provide:

<li><a href="LICENSE">LICENSE</a></li>
<li><a href="INSTALL.html">INSTALL</a></li>
<li><a href="DEPLOYMENT.html">DEPLOYMENT</a></li>
<li><a href="user_manual.html">User Manual</a></li>

The shared template offers only "All Library Apps", "Home", and the two
conditionals. So adopting means either dropping those four links or giving the
shared template a way to take extra nav entries from metadata — which is
probably the more useful outcome for every repository, and would be a change in
caltechlibrary/workflows.

When this is done

• docs.yml drops the template: input and uses the shared default
• pandoc/page.tmpl, pandoc/css/code-blocks.css and src/copyToClipboard.js
  are deleted
• pages gain a charset declaration, a viewport tag and real per-page titles

Depends on the theme-assets change in caltechlibrary/workflows landing first.