fix(doc-util): escape dotted keys

[Duologic]

Attempt to fix #16 in doc-util rendering, this is a bit more complex than I anticipated, I've used this example to test various places where dotted keys can be.

local d = import './doc-util/main.libsonnet';

d.render(
  {
    '#': d.pkg('a', 'a', 'a',),
    '#has.a.period': d.val('help1'),
    'has.a.period': null,
    'more.period.keys': {
      '#': d.pkg('more.period.keys', 'c', 'more.period.keys',),
      'also.has.period': {
        '#period': d.val('help2'),
        period: null,
        '#has.a.period': d.val('help3'),
        'has.a.period': null,
      },
    },
    has: {
      a: {
        '#period': d.val('help4'),
        period: null,
        '#has.a.period': d.val('help5'),
        'has.a.period': null,
      },
    },
  }
)

I also don't think people should use dotted keys as they are confusing, but that is another discussion.