render a latex environment input as a paragraph

[likanzhan]

Sometimes, the output of a @example block is itself a latex environment, such as a DataFrame.

```@example
using DataFrames
df = DataFrame(i=1:4, y='A':'D', z=5:8);
df
```

In this case, some special characters, such as & and \, are not need escaped, except for the linebreak symbol \\.
This pr intends to do this.
This is related to issue #1346.

[mortenpi]

It's not immediately clear to me what exact issue this fixes. Could you add a small test case to e.g. the latex_simple tests for this? tests_latex.jl builds and tests the LaTeX tests, but you do have to instantiate the test environment.

[mortenpi]

The [begin] syntax was added in 1.4, so this would have to be rewritten as e.g. first(md.content) to be compatible with 1.0.

[mortenpi]

Elements of md.content are not guaranteed to be strings, so this implementation may throw an error: https://travis-ci.org/github/JuliaDocs/Documenter.jl/jobs/717872752#L320-L329

[mortenpi]

I took the liberty of updating the tests a bit, so the last inline comment is now attached to the wrong place. However, in the examples:

1. The Table() does work, but does not render correctly. I think it should be handled here, but isn't for some reason. So this would be nice to have a fix for.
2. The at-raw block here should render as a table (and does render correctly).
3. The text example below should not render as a table, because it is semantically just text. To include raw LaTeX, you should use an at-raw block. I am pretty sure that the changes to the Markdown.Paragraph method are unnecessary.

This is how the examples render with master (since this branch currently errors): DocumenterLaTeXSimple.pdf