Expand on validation section and add more examples

[GokceBaykan]

As part of their GRESB API integration process, data partners are asking for an overview of all the rule interdependencies across asset data fields. We cannot provide this set of rules and therefore we should at least make sure to expand the validation section with information and practical cases (from RC, Energy, etc.) on how the reporting errors are addressed incrementally.

[GokceBaykan]

Input from @joeyhorst for what we need to include within the examples:

• the longest validation messages
• a validation message with many references to variables with tool tip expansion
• validation messages which depend on a number of preconditions
• simple validation (i.e. presence validation)

[joeyhorst]

For validation hierarchy we could refer to https://gresb-prd-public.s3.amazonaws.com/2021/RE_Documents/Real_Estate_Asset_Portal_Validation.jpg from https://www.gresb.com/nl-en/asset-portal-guide/.

To support tooltips for variables, we can refer more clearly to https://github.com/GRESB/api-docs/blob/master/data/dictionary.yml to use as variable descriptions. With an additional screenshot of how we deal with it in our own platform.

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Most of all ensure that the examples in https://github.com/GRESB/api-docs/blob/master/source/includes/_assets.md don't have empty validation:

    "_validations": {
        "errors": {}
    }