Implement REF_ALTTEXT macro for DDox

[JakobOvrum]

For DDoc,

$(REF_ALTTEXT writeln, File.writeln, std, stdio)

expands to

<a href="std_stdio.html#.File.writeln">writeln</a>

which relies on the fact that File.writeln is part of the URL. To provide identical functionality for DDox, it would have to expand to:

a href="/library/std/stdio/File/writeln>writeln</a>

REF handles this by expanding to std.stdio.File.writeln and letting DDox recognize it as a symbol path, but I'm not aware of an equivalent feature in DDox that works like REF_ALTTEXT.

This patch makes it expand to:

writeln (std.stdio.File.writeln)

It's not ideal but it's an improvement over the broken output it currently generates.

[CyberShadow]

LGTM though it doesn't look like it's used anywhere?

[aG0aep6G]

DDOX recently got a macro DDOX_ROOT_DIR (commit). With that it should be possible to define the DDOX REF_ALTTEXT similarly to the Ddoc one.

That would require updating DDOX to 0.13.3 in the dependencies of dpl-docs (here).

[JakobOvrum]

@CyberShadow, it's used in a couple of PRs, including dlang/phobos#3911. It's intended as a companion to REF replacing all the _NAMED macros... but it's not a straight upgrade considering the less-than-ideal DDox implementation. Right now those macros simply link from DDox to DDoc.

@aG0aep6G, the root directory isn't the problem - we have ROOT_DIR for dlang.org. There's no way to "replace" dots with slashes using DDoc macros.

[aG0aep6G]

@aG0aep6G, the root directory isn't the problem - we have ROOT_DIR for dlang.org. There's no way to "replace" dots with slashes using DDoc macros.

Ah, right. Sorry for the noise. This sucks.

[JakobOvrum]

Yeah, it sucks. DDoc macros can only have one variadic component and we have no way of "splitting" one argument into many.

Markdown has [text here](link here). There's been some murmuring about Markdown before and maybe it would provide a better way out. For now the implementation in this PR is all I got.