Add models/processes to overview

[Thom1729]

Add a short introduction to the data models and processes in the language overview. The new descriptions are meant to be formally correct, but high-level and incomplete. This is in preparation for expanding the model and process sections into full individual chapters.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[Thom1729]

https://spec.yaml.io/pr-242

[Thom1729]

I generally agree. I think that in this case there is value to having a quick, formal-but-not-complete listing of the information models and processes before the detailed descriptions in the chapters. This PR adds text, but I'm hoping that having a concise introduction here will help us to remove redundant text elsewhere.

[ingydotnet]

I'd like to propose slowing down on shaping the 1.3 spec until we know more clearly what we are intending to specify.

In my current opinion, the successful rollout of YAML 1.3 depends on several things including:

• The YAML RFC process
• The YAML test suite for 1.3 processes
• The 1.3 Spec document
• The 1.3 implementer guides
• The 1.3 YAML user guides
• The 1.3 reference grammar(s)
• The 1.3 reference implementations
• Various feature specs like ypath and yfun that we want to promote
• The 1.3 scheme (YAML processor config) format
• The 1.3 standard library

etc.

I feel like at this point we need to flesh out these ideas to get a clearer vision of what 1.3 will be, before we spend much more time of the spec.

For instance re #241, I'd like to see published documentation that replaces anything we want to remove.

Let's hold off on merging this #241 and #242 for now...

We can continue to make low hanging, non-contentious modifications to the productions, but the overall shape of the informative parts will be more obvious after we make progress on the other fronts.