autodoc: decltests for files-as-a-struct don't show up

[squeek502]

Example:

zig/lib/std/heap/FixedBufferAllocator.zig

Line 161 in 86d3546

test FixedBufferAllocator {

doesn't show up in the FixedBufferAllocator docs:

https://ziglang.org/documentation/0.14.0/std/#std.heap.FixedBufferAllocator

[squeek502]

Looking into it a little bit, I think there are two problems:

1. renderNamespacePage in main.js does not call renderDocTests
2. w.file.get().doctests does not get the doctest added, that only happens for function names

The first one is easy to fix, unsure how to go about fixing the second, since I'm not sure what name to lookup to find the associated doctest.

EDIT: One potential fix using the stem of the filename submitted here: #23615