Fix headers in LaTeXWriter

[odow]

The same general fix as #1417, with credit to @cserteGT3

Rather than testing the LaTeX page structure, I just added tests for the structure returned by files.

Closes #1416
Closes #1417

Here's the change from my MathOptInterface-inspired layout (jump-dev/MathOptInterface.jl#1638):

Old

New

[fredrikekre]

Can you comment on Mortens comment here: #1417 (comment) ? Is that relevant to this PR?

[odow]

Nested subparts work as expected.

import Documenter
Documenter.makedocs(
    sitename = "bug",
    pages = [
        "Introduction" => "index.md",
        "Tutorials" => [
            "tutorials/tutorial1.md",
            "Sub-part" => [
                "tutorials/tutorial2.md",
                "Release notes" => "release.md",
            ],
        ],
    ],
    format = Documenter.LaTeX(),
)

produces

"Tutorials" is the \part, "Sub-part" becomes the \chapter, and the level-1 header in tutorial2 becomes the \section

[odow]

I think this is more a bug-fix rather than a feature change.

The underlying problem is that "Introduction" => "index.md" gets inferred as a Pair{String,Any} because it an element of Vector{Pair{String,Any}}.

[fredrikekre]

Okay great. Can you add a note in the changelog and update the version in the toml file?

[odow]

Yip will do.

[cserteGT3]

@odow Thanks for taking care of this one! 🚀