Fix non-text/plain output for Markdown and LaTeX

[fredrikekre]

Fix non-text/plain output for Markdown
and LaTeX output by capturing several
output representations and let each writer
choose which one to render,
fixes #916, fixes fredrikekre/Literate.jl#49.

[mortenpi]

Hmm, the images on the man/tutorial.md page in test/examples/ should be built via this, right? In the Markdown output I get things like this:

```julia
GIF(joinpath(dirname(pathof(Documenter)), "..", "test", "examples", "images", "logo.gif"))
```


```
Main.ex-inlinewebpgifjpeg.InlineWEBPGIFJPEG.GIF("/home/mortenpi/Julia/JuliaDocs/Documenter/src/../test/examples/images/logo.gif")
```

I suggest we put this in as an enhancement in 0.22 even though it might technically be eligible for a backport?

[fredrikekre]

I suggest we put this in as an enhancement in 0.22

Definitely.

[fredrikekre]

Hmm, the images on the man/tutorial.md page in test/examples/ should be built via this, right?

Yea I guess, I just didn't enable gif output for markdown files, but should be the same as e.g. png's I guess. It took the text/plain path here (which is still better than the current status which errors since the markdown writer can not handle RawHTML).

[mortenpi]

A few more thoughts:

• It should not be a problem to also support WebP images in MarkdownWriter in the same way.
• It should actually be fine to have arbitrary HTML in the outputted Markdown -- MkDocs should handle it without issues (Documenter outputs some anyway, e.g. the anchor tags). So the <img src="data:..."> approach should also work for MarkdownWriter too. It should also be possible to output text/html. Needs to be tested though.

To test the MkDocs build, docs_dir in test/examples/mkdocs.yml needs to be changed to builds/markdown.

[tkf]

Doesn't it make sense to prefer text/latex in latex writer? I don't use latex writer myself so I don't know what's the optimal order, though.

[fredrikekre]

I thought it made sense to prioritize more rich formats, such as images, before latex 🤷‍♂️. I don't think it is very commong for things to be showable in both formats so probably doesn't matter.