Add semantic ids/anchors to section

... so that I can send a link to a particular section/subsection on the website and a recipient that follows the link gets right to the section/subsection I wanted them to.

Current situation:
- bigger sections have numeric id
- subsections (e.g. FrontEnd course subsection) do not have any referrable id

Inspect the website sections and subsections that currently use numeric ids, including the FrontEnd course subsection. Add stable, semantic anchors so each section and subsection can be linked directly, then verify that links open at the intended location.

Written by the indexing model from the issue text.