All TODOs from documentation

[jpmckinney]

These are in HTML comments on the following pages:

• getting_started/building_blocks.md: Add section on mapping - including links to mapping tools
• getting_started/index.md: You can find samples of OCDS data, and information on using OCDS in the implementation handbook.
• schema/index.md: Consider adding diagram here
• schema/reference.md: Add example (milestones)

[jpmckinney]

getting_started/building_blocks.md: Add section on mapping - including links to mapping tools

This is covered in the Map phase of the new Guidance section.

You can find samples of OCDS data, and information on using OCDS in the implementation handbook.

We no longer provide real sample data in the sample-data repository (only fictional data), as our strategy is to develop worked examples as a more useful way to explain how to model data in OCDS (rather than leave it up to the implementer to reverse-engineer from existing data, which is error-prone, considering much data has conformance issues).

Specific guidance on using OCDS data is covered by the OCP website and is not in scope for the OCDS documentation. https://www.open-contracting.org/data/data-use/

schema/index.md: Consider adding diagram here

It's not clear what diagram was intended for this page. #991 is the more general issue about diagrams and other visuals.

schema/reference.md: Add example (milestones)

See #949. I created an issue to link to examples from reference pages #1018.