**What type of PR is this?**

> Documentation

**What package or component does this PR mostly affect?**

> docs

**What does this PR do? Why is it needed?**

Previously, extend.md and repository.md were generated with Stardoc.
These files are explanations rather than references, and the doc strings
are in files that weren't meant to be imported. Newer versions of
Stardoc add load statements for the internal .bzl files that we don't
want.

Instead:

- Added reference.md, generated from def.bzl. This contains reference
documentation for all importable symbols.
- Removed extend.bzl. The explanation here is useful, but we can edit
extend.md directly and refer to symbols in reference.md.
- Removed repositories.bzl and repositories.md. This only contained
reference documentation for go_repository without a separate
explanation, so we can use that instead.

No change to extensions.bzl or extensions.md, which provide reference
documentation for go_deps.

**Which issues(s) does this PR fix?**

Fixes #763

**Other notes for review**