merge content from main branch into documentation gh-pages branch to reduce confusion

[MathewBiddle]

This repo is a little confusing with the two divergent branches. I think the content in main could be easily moved into gh-pages and the website can be updated to include the additional details provided in main.

Here's what I'm thinking:

• Move https://github.com/ioos/ioos-atn-data/tree/main/notebooks to https://github.com/ioos/ioos-atn-data/tree/gh-pages/_docs/notebooks
• Create a new folder in https://github.com/ioos/ioos-atn-data/tree/gh-pages/_docs for data/templates where we will put the .cdl templates.
• Create another folder in https://github.com/ioos/ioos-atn-data/tree/gh-pages/_docs for crosswalk/ and move https://github.com/ioos/ioos-atn-data/blob/main/crosswalk/ATN_nc_to_DwC_OBIS_mapping.xlsx there.
• Create a new webpage for https://github.com/ioos/ioos-atn-data/blob/main/README-ArchiveSOP.md and link to it using the sidebar
• Create a new webpage for https://github.com/ioos/ioos-atn-data/blob/main/README-ATN2DWC.md and link to it using the sidebar
• Update https://github.com/ioos/ioos-atn-data/blob/gh-pages/README.md to reflect the changes, if needed.
• Move GitHub Actions to gh-pages branch https://github.com/ioos/ioos-atn-data/tree/main/.github. Check paths in all actions.
• Make gh-pages the default branch (in Settings > General > Default Branch).
• Check webpages to ensure everything is working correctly.
• remove main branch.