Deprecate the almanac custom metric

[rviscomi]

The almanac custom metric was created as a catch-all for any new data needed for the 2019 Web Almanac. In subsequent editions, we organized new metrics in "chapter-specific" files. Some almanac metrics are obfuscated by chapter/metric identifiers (my mistake) and some are made redundant by newer custom metrics.

The Web Almanac is on hold this year and its future is unclear. We should decouple the metrics from the project by relocating all of the custom metrics into the most closely related "chapter-specific" files. There should be no loss of data in this migration.

For example, almanac.meta-nodes would be a better fit in the markup custom metric. 10.5 is the name for a structured data custom metric, so it would be a better fit in structured-data. If that custom metric already includes the same functionality, we can drop it.

To help users migrate older queries to newer datasets, we should document where all of the almanac custom metrics went.