Add search bar for documentation

Here's how I think this could be implemented:

Modify site-src/document-content.crow to provide an ID for each function/type/spec, so that a link can refer to the particular function. E.g.: /doc/crow/compare.html#equal.
- It's not important to distinguish overloads (or a type and function having the same name) since they should be close together anyway
Modify site-src/document.crow walk over all-modules and collect search terms.
- See module.crow for the type definition of a module. Just walk over the tree that represents a module and collect search terms.
- Split doc comments into words to get search terms. script/lint.crow already has a function for parsing words, so that could go in a module shared by both script/lint.crow and the new code.
- Omit common words like "the" from the search terms.
Store the search terms in a file doc/search.json (exact format can be whatever). Load that in the browser and use that to implement a search box.
- Put this at the top of the navigation pane on the left.
- Based on a brief search fuse.js seems to implement this kind of functionality. Other libraries could be used.
An alternative would be to use the results of crow doc directly. The script site-src/site.crow starts by running a crow doc command. This generates 539KB of data though; it includes things like parameter information that isn't relevant for search. I think if we parse out the search terms ahead of time we can get a smaller format.

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