Front Matter to override docs lastUpdate date/author

[slorber]

Have you read the Contributing Guidelines on issues?

• I have read the Contributing Guidelines on issues.

Motivation

Suggested in #7229 (reply in thread)

We should allow docs frontMatter that permits to override the "last updated" data that we generally read from Git.

Git is not always available at build time, but users should still be able to manually fill those data

lastUpdate:
  at: 1539502055
  by: "Author"

Front matter validation could accept the date as timestamp (seconds or millies?), or ISO/parseable string? (+ JS Date because Yaml may automatically do the conversion)

This is a relatively simple issue, good for external contributors

Self-service

• I'd be willing to do some initial work on this proposal myself.

[Josh-Cena]

Duplicate of #5691

[Josh-Cena]

Let's discuss it there instead. The API is slightly different, but not a big deal