proof of concept of stage1 doc generation

[andrewrk]

This commit adds -fgenerate-docs CLI option, and it outputs:

• doc/index.html
• doc/data.js
• doc/main.js

It works without a server, e.g. you can use the file:/// URL in a web browser.

In this strategy, we have 1 static html page and 1 static javascript
file, which loads the semantic analysis dump directly and renders it
using dom manipulation.

Currently, all it does is list the declarations. But there is a lot more
data available to work with. The next step would be making the
declarations hyperlinks, and handling page navigation.

Another strategy would be to generate a static site with no javascript,
based on the semantic analysis dump that zig now provides. I invite the
Zig community to take on such a project. However this version which
heavily relies on javascript will also be a direction explored.

I also welcome contributors to improve the html, css, and javascript of
what this commit started, as well as whatever improvements are necessary
to the static analysis dumping code to provide more information.

See #21.

To see this for the standard library, try this:

./zig test ../lib/std/std.zig -fgenerate-docs --output-dir zig-cache/

Then have a look at zig-cache/doc/index.html.