Download and Checksum were addressed on the template.

example shows Download, but Checksum done in same fashion.
Before:

<div class="section">
   <h3>Download</h3>
    <ul> 
    </ul>
</div>

After:

<div class="section">                             
    <h3>Download</h3>
     <p> Downloads are not available for this article.</p>
</div>

Autogenerated TOC addressed in the JS

This removes the "jump to" section completely.

To Remove Empty Sections or Not to Remove...

For Checksum and Download, I have left the section there but added a statement that there are none available. This is because these are standard sections that a reader might 'look for' and so they should not be hidden but state clearly that they are empty.

But for the automated table of contents, I have removed the section entirely when it is empty. This is because the table of contents listing the sections does not make any sense in the context of an article without contents - and a reader would not look for it when there is no contents to navigate. To follow the pattern above with downloads and checksums would add to user confusion not prevent it.

Closes #3911

@joemull please note response #4293 (comment)

for testing, I have tried all combinations of:

• each of the 3 themes
• wide or narrow windows
• articles with or without galleys
  Anything I've missed?

Following this, it seems both Clean and Material use the same toc.js. So now updating and rechecking that.

Summary of Changes:

• I have made the changes requested.
• I retested the new functionality (as described in previous comment) and updated where needed.
• I found that the Material and Clean themes use the same toc.js asset, which is in the Material directory, this seems to be a carbon copy of the toc.js in the OLH assets. I updated that toc.js and added the 'toc-section' id to the article.html files so that the toc-section can be hidden when no headings have been found while generating the toc.
• The material theme tested fine, because it has other functionality which suppresses the TOC for short articles, such as the test example. The TOC card is not visible until one scrolls further down the page, and the page is too short to reach that point which is why it appeared to work correctly before the material toc.js update. I decided that for consistency, I would update the material article.html to be consistent with the other two themes, despite this... which means if later updates are made to the material theme which change when the TOC card is visible, such that it is visible on shorter pages, the purpose of this issue PR, to hide the TOC when there is no content for it, will then function consistently with the other two themes. But I am unable to directly test it working - I can note that it is exactly the same as in the Clean theme and that does work.