Various improvements to the docs

[waldyrious]

This PR includes several changes to the docs

• Fix a couple typos in the docs (including one that broke a reference link)
• Remove backticks from @ref links to macros (so the links will actually work instead of point to a 404 page)
• Normalize whitespace in docs (two spaces between sentences; soft wrapping in .md files; semantic wrapping in .jl files)
• Add a full reference documentation page (via the autodocs macro)
• Add docs/build/ to the .gitignore file

Each change above corresponds to a separate commit, so they can be reviewed independently, and if any of them are not desirable, they can be easily removed from the branch.

(This PR is a follow-up to #26.)

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[waldyrious]

A couple questions from the docs build log:

1. Can we fix the two warnings about duplicate docs found for Gettext.bindtextdomain and Gettext.textdomain? I wonder why they were defined as docstrings attached to loose tokens separate from the actual functions. Is that a Documenter.jl idiom?

2. Could we activate previews for docs changes such as this? The build log reports that the push_preview keyword argument to deploydocs was not set to true. Is it just a matter of turning that on, or would we need additional configuration?

[stevengj]

Mostly LGTM. I'm not sure if the @autodocs section is needed, though, however since we already have @docs blocks for all of the docstrings?

(And if the same docstring is included in a manual @docs block and an @autodocs block, which one will a @ref link point to?)

I defined the docstrings for textdomain and bindtextdomain as "loose" docstrings because each function had two methods and it wasn't clear which method to attach the docstring too. That's a part of Julia's docstring syntax, not specific to Documenter.jl

Why are we getting a warning?