Cleaning up the issue URLs

[kerimoyle]

Some thoughts about Issue urls.

• It's best practice to have human-friendly URLs -> we should use a string error code in the URL rather than the specification heading number. This is addressed in #813
• Use the spreadsheet about libCellML validation coverage to decide what the mapping should be from issue codes to sections. NB this is needed so that we have coverage, but might not need to address child/parent points.
• Update the *.inc files to replace the current .. container:: issue-1-2-3 number-based tags with .. container:: issue-SOME-ISSUE-CODE accordingly.
• Remove the static mapping in the cellml-specification repo in the page.html file which handles the bookmark/highlighting.