Add an unified .Site.Config with a services section

[bep]

With the recent privacy config addition, we can do:

.Site.PrivacyConfig.Instagram

Working on making "privacy friendly" editions of some of the shortcodes, we need to add some additional service-specific config. This is currently not possible without too much ceremony. It needs to be available from shortcodes and we don't want to add another top level .Site field.

Back when Hugo was a youngster, it probably made sense to put DisqusShortname on the top level site config, but now that looks sub-optimal.

We can fix that while keeping the old sites working as before.

It also makes sense to keep the privacy config as one copy-and-paste-friendly entity. So I suggest that we make it so:

.Site.Config.Privacy.Instagram
.Site.Config.Services.Instagram

Would give two config objects that both relate to Instagram.

Also, we should make it so

.Site.Config.Services.Disqus.Shortname

Points to the same value as .Site.DisqusShortname (keeping the old, but eventually remove it from the docs).

Also consider:

googleanalytics

There are other candidates, but those will have to wait.

[github-actions]

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