Add necessary files and new build targets for ddox based HTML documentation

This pull adds two new make file targets "apidocs-prerelease" and "apidocs-release" to posix.mak as alternatives to the normal HTML API docs targets. It also has a number of small visual tweaks that have been brought up during the informal review of the docs.

The documentation generator assumed to be located in the [tools repository](https://github.com/D-Programming-Language/tools/pull/43).

Building the docs:
```
LATEST=2.061
make -f posix.mak html
make -f posix.mak apidocs-release
```

Note that the generator currently fails at some point in `std.digest.sha`. This is an issue of either DMD or DDOX and not directly related to this pull. It will be fixed shortly.