Improve api_docs md->html script to eliminate manual steps

@DavidHuie @arionhardison This change eliminates all manual steps and makes the markdown conversion a breeze again.

* Set css class and border size for html tables.
* Move the generated table header row down to the body. The `table-docs` css class will make the first row look like a header. With an explicit table header we would effectively get 2 header rows.

[Here](http://nationbuilder.com/custom_fields_api) is a page with data table.

* Do not remove the `h1` heading except the very first one. The first row is usually the same as the page name in NationBuilder and the CMS will display it. The other headings contain valuable information and should not be stripped.

[Here](http://nationbuilder.com/contacts_api) is a page with multiple `h1` headings that were stripped out before.