sso_*: various documentation additions

[Jusshersmith]

• As described in https://github.com/buzzfeed/sso/issues/302, we're pointing to a file that no longer exists.

• In the quickstart, UPSTREAM_CONFIGSFILEshould be UPSTREAM_CONFIGFILE (note the 'S' has been removed in 'CONFIGFILE'). Unless corrected, the quickstart results in Misdirected Request. https://github.com/buzzfeed/sso/blob/cb8d5233440284f1d264ff901f39cc3e3fa53e4d/internal/proxy/configuration.go#L369

• Some general touchups of documentation in docs/sso_config.md

• Creation of adocs/sso_proxy_config.md file, containing all available configuration variables for sso_proxy (the equivalent sso_auth file is here. For reference, a lot of these descriptions existed once upon a time here: https://github.com/buzzfeed/sso/blob/d2e1ee5484b62a2226f718ee760782050139ca1f/internal/proxy/options.go#L19-L53

[codecov[bot]]

Codecov Report

Merging #315 (8071ac0) into main (b008111) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #315   +/-   ##
=======================================
  Coverage   62.73%   62.73%           
=======================================
  Files          58       58           
  Lines        4286     4286           
=======================================
  Hits         2689     2689           
  Misses       1382     1382           
  Partials      215      215           

Impacted Files	Coverage Δ
internal/proxy/configuration.go	89.80% <ø> (ø)

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