Generate documentation stubs from Javadocs

illuaminate does not handle Java files, for obvious reasons. In order to
get around that, we have a series of stub files within /doc/stub which
mirrored the Java ones. While this works, it has a few problems:

 - The link to source code does not work - it just links to the stub
   file.
 - There's no guarantee that documentation remains consistent with the
   Java code. This change found several methods which were incorrectly
   documented beforehand.

We now replace this with a custom Java doclet[1], which extracts doc
comments from @LuaFunction annotated methods and generates stub-files
from them. These also contain a @source annotation, which allows us to
correctly link them back to the original Java code.

There's some issues with this which have yet to be fixed. However, I
don't think any of them are major blockers right now:

 - The custom doclet relies on Java 9 - I think it's /technically/
   possible to do this on Java 8, but the API is significantly uglier.
   This means that we need to run javadoc on a separate JVM.

   This is possible, and it works locally and on CI, but is definitely
   not a nice approach.

 - illuaminate now requires the doc stubs to be generated in order for
   the linter to pass, which does make running the linter locally much
   harder (especially given the above bullet point).

   We could notionally include the generated stubs (or at least a cut
   down version of them) in the repo, but I'm not 100% sure about that.

[1]: https://docs.oracle.com/javase/9/docs/api/jdk/javadoc/doclet/package-summary.html

doc/stub/http.lua

@@ -198,32 +198,3 @@ function websocket(url, headers) end
 -- @tparam[opt] { [string] = string } headers Additional headers to send as part
 -- of the initial websocket connection.
 function websocketAsync(url, headers) end
-
---- A websocket, which can be used to send an receive messages with a web
--- server.
---
--- @type Websocket
--- @see http.websocket On how to open a websocket.
-local Websocket = {}
-
---- Send a websocket message to the connected server.
---
--- @tparam string message The message to send.
--- @tparam[opt] boolean binary Whether this message should be treated as a
--- binary string, rather than encoded text.
--- @throws If the websocket has been closed.
-function Websocket.send(message, binary) end
-
---- Wait for a message from the server.
---
--- @tparam[opt] number timeout The number of seconds to wait if no message is
--- received.
--- @treturn[1] string The received message.
--- @treturn boolean If this was a binary message.
--- @treturn[2] nil If the websocket was closed while waiting, or if we timed out.
--- @throws If the websocket has been closed.
-function Websocket.receive(timeout) end
-
---- Close this websocket. This will terminate the connection, meaning messages
--- can no longer be sent or received along it.
-function Websocket.close() end