Suppress writing of XML declarations

[kimikage]

In case of HTML output format, SVG images can be embedded as outputs of @example blocks and so on. However, show(io::IO, ::MIME"image/svg+xml", object) may write an XML declaration. In most major web browsers, such invalid XML declarations have no effect on the rendering, but they may cause warnings.

There are roughly two possible measures:

1. Temporarily write the output to a buffer and then strip the XML declaration
2. Suppress show method from printing the XML declaration

The former is a straightforward method, but it is not efficient. The latter is more efficient, but currently there are no interfaces for the suppression. Therefore I suggested defining a new IOContext property. (See: https://github.com/JuliaLang/julia/issues/35586)

If the latter approach (2.) works, there are two possible strategies.

• (2-a) Build the consensus in the Julia community first, then support the property in Documenter.jl
• (2-b) Support the property in Documenter.jl first, and wait for it to become the de facto standard

Any thoughts?

[mortenpi]

Two brief thoughts on the Documenter side of this:

• To pragmatically solve the SVG issue, I would be perfectly happy with some best-effort regexing and removal of the <?xml declarations on Documenter's side. I don't think we would need to be particularly concerned about the efficiency, or even aim for it being 100% correct all the time.
• If, however, an IOContext convention does arise, then Documenter should definitely adopt it.

[kimikage]

I believe removing XML declarations is not an urgent task. If we decide to implement it, I'm not sure which layer should do it. (So, I cannot submit the PR immediately.:sweat_smile:)