Reorder the pkgdown function reference page

[capnrefsmmat]

Here's the current version: https://cmu-delphi.github.io/epidatr/reference/index.html

We choose the ordering in _pkgdown.yml:

epidatr/_pkgdown.yml

Lines 35 to 64 in ee952c5

	reference:
	- title: Configuration and utilities
	desc: Set API keys and handle API data types
	- contents:
	- get_api_key
	- avail_endpoints
	- epirange
	- timeset
	- title: Control Caching behavior
	desc: Configure an optional persistent cache
	- contents:
	- set_cache
	- clear_cache
	- disable_cache
	- cache_info
	- title: Make API requests
	desc: Query Delphi Epidata endpoints
	- contents:
	- epidata_call
	- fetch_args_list
	- has_keyword("endpoint")
	- -starts_with("pvt_")
	- title: Make requests to private API endpoints
	desc: These endpoints require additional authorization to use
	- contents:
	- starts_with("pvt_")
	- title: Autocomplete helpers
	desc: Objects that can autocomplete API signals names for faster exploration
	- contents:
	- covidcast_epidata

I'd put important things first, and rarely used stuff at the bottom. For instance, caching control and private API methods should both end up at the bottom. Perhaps:

1. Make API requests
2. Configuration and utilities
3. Autocomplete helpers
4. Make requests to private API endpoints
5. Control caching behavior (with the word casing fixed on this one too)

I figure that once someone gets past the Getting Started vignette and sets their API key, most of their use of the reference will be to find specific API endpoints, so this puts the important things first.