Internals docs

[dsweber2]

closes #108
In the process of doing this, I noticed that the epidata_call docs are in a weird state. On the reference page, there's a mass of functions all together create_epidata_call() fetch() fetch_tbl() fetch_classic() request_url() with_base_url() which point at that page. Several of these are internal only, and some were actually moved from external to strictly internal. The second commit in the pull request may deal with that

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (dev@9282f34). Click here to learn what that means.
The diff coverage is n/a.

❗ Current head 9aa0cf2 differs from pull request most recent head f28ae6e. Consider uploading reports for the commit f28ae6e to get more accurate results

❗ Your organization is not using the GitHub App Integration. As a result you may experience degraded service beginning May 15th. Please install the Github App Integration for your organization. Read more.

@@          Coverage Diff           @@
##             dev     #125   +/-   ##
======================================
  Coverage       ?   94.61%           
======================================
  Files          ?        8           
  Lines          ?     1169           
  Branches       ?        0           
======================================
  Hits           ?     1106           
  Misses         ?       63           
  Partials       ?        0           

[dsweber2]

actually confirmed that the last one displays create_epidata_call() fetch() with_base_url() in the reference section

[dshemetov]

I wish we had a way to preview these in PRs...

Otherwise, these specific changes look good. Since this isn't a final doc draft, I'm just going to let this go through and we can see what the docs look like as the CI builds them. We may want to remove even more functions from the docs. There are also plenty of other changes to make for #117