Clarify core curriculum

[cgross95]

As mentioned in #54 and #74, the lesson page is difficult to parse. This could be improved by outlining the core curriculum for an HPC Carpentry workshop like what's done on the Organization README. I personally like the table format used by the other Carpentries.

I could see two different places to add a table and short description:

1. To the current lesson page:
   • Pros: Brings structure to the page; keeps the same URL
   • Cons: Adds more to an already dense page; the Software Carpentry shell lesson does not fit into any of the three categories on the community-lessons page
2. To a new page:
   • Pros: Cleaner; clarifies what's being actively maintained(?)
   • Cons: Duplicates lesson listings

I personally lean towards option 2 for simplicity, but an overhaul of the lesson page could make 1 more attractive. In particular, the Library Carpentry's presentation is pretty clear and something similar could be adapted. I imagine one table with the Core Curriculum and another with the HPC Carpentry lessons as an Extended Curriculum along with "status" tags to clarify retired or no-longer-maintained lessons.