* Move `load_tera` to `templates`
I don't know if this is a good place for it, conceptually. I'm moving it
there because I need to use it from `templates`, and `templates` can't
depend on `site`, because there's already a dependency in the opposite
direction.
* Load templates in `markdown` filter
This enables the `markdown` filter to handle shortcodes, as long as
those shortcodes don't access any context variables.
Addresses #1350
* Update documentation of `markdown` filter
* Only load templates for `markdown` filter once
* Clarify `markdown` filter documentation
This is a lightly edited version of what @southerntofu suggested.
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