reorg for new users exploring data model

[sbailey]

This PR is a work in progress to reorganize the data model presentation to be more oriented towards new users exploring the datamodel and directory tree, rather than expert developers:

• Top level landing page splits most commonly used directories (e.g. spectro/redux/ and target/) from historical/expert directories (like protodesi/)
• Non-directory things (like mask bits) get their own bullet list section instead of being mixed in with directories.
• Landing page and descriptions emphasize the default path relative to $DESI_ROOT rather than the expert-level environment variables (e.g. $DESI_ROOT/spectro/redux over $DESI_SPECTRO_REDUX).
• The environment variables that point to subdirs under $DESI_ROOT are moved to a separate page rather than being the primary starting point, since most new users won't know what $DESI_SPECTRO_REDUX is how how it relates to the EDR root directory.

So far I've just updated the landing page and the $DESI_ROOT/spectro/data tree (while admitting that isn't the most commonly used dir by end-users). I'm posting this PR as a WIP now so that others (especially @weaverba137) can look and comment if needed before I spend too much more time adding similar changes to the DESI_SPECTRO_REDUX tree.

Other than the addition of envvars.rst, this PR (and future updates to it) only modifies index.rst files, not the core directory structure nor the data model files themselves.

[coveralls]

coverage: 100.0%. remained the same when pulling 4cda055 on exploring into 521f87e on main.

[weaverba137]

This looks good. I had only some minor suggestions that should be easy to implement.

[sbailey]

@weaverba137 thanks for the pre-review; this is now ready for final review. The main updates are to the landing page and the pages directly linked from the landing page. In the interest of time I'm cutting myself off descending to deeper levels of linking, though that could use some more cleanup in the future.

[weaverba137]

Looks good to merge now.

[sbailey]

Thanks. I also addressed the csv quoting issues in column_descriptions.csv to fix #174 so that github displays it nicely, but I didn't go all the way to #175 to render it into rst automatically.