Cleanup some debris incurred by !519

Background

Recently a workspaces example was added to the user manual, in MR !519 (merged), which has a few problems that need to be fixed.

Task description

• Fix the .. note:: statement at the beginning of the workspaces tutorial, the link is not rendering and the note box is not showing.
• Fix the cheat invocation of bst with appended shell commands separated by ;, which is just not supported and doesnt even work when building the docs (see the online tutorial and notice that the output of the bst shell invocation does not show the expected "We can use workspaces" text).
• Move the stored session html to doc/source/sessions-stored, where the sessions are stored

Alternatively to moving the stored session html files to the place where we are storing them, one could attempt to refactor the Makefile and move all the stored session html files to doc/source/session, once one has ensured that docs build under every circumstance, and rebuild properly when BST_FORCE_SESSION_REBUILD is specified.