Enable generating a table of contents in Stardoc

[ahumesky]

Adds a bool generate_table_of_contents to stardoc() to enable table of contents. This defaults to False for backwards compatibility. Adds a Label table_of_contents_template to stardoc() to specify the vm template.

The default table of contents template looks like this (from the test):

Rules

• my_rule

Providers

• MyFooInfo
• MyVeryDocumentedInfo

Functions

• check_sources
• returns_a_thing

Aspects

• my_aspect
• other_aspect

Repository Rules

• my_repo

Module Extensions

• my_ext

[ahumesky]

The presubmit wants me to run buildifier on stardoc/private/stardoc.bzl but this results in a large diff of unrelated changes -- should we ignore this, accept the large unrelated diff, or run buildifier on the whole repo and then sync this?