Document metadata input and validation procedures

[mpadge]

Likely best as a separate vignette. This needs to include:

• [x] Descriptions of how the JSON schemas work, and how they are validated. (Without necessarily directly referring to them at all?)
• [x] Demonstrations of the kind shown in this issue comment, where metadata fields put in the wrong items generate informative error messages

• [ ] Clear tables or similar structures explicitly showing internal mappings of all metadata items. These could likely be dumped in the vignette directly from the internal get _service_translation() function: https://github.com/ropenscilabs/deposits/blob/6fdbf5a461ca63f6bdd16aca56c3d80313726bc4/R/metadata-translate.R#L90-L93 This returns output like this:

  library (deposits)
  service <- "zenodo"
  translations <- deposits:::get_service_translation (service)
  head (translations)
  #>               source      path       target
  #> 1           abstract /metadata  description
  #> 2       accessRights /metadata access_right
  #> 3      accrualMethod /metadata        notes
  #> 4 accrualPeriodicity /metadata        notes
  #> 5      accrualPolicy /metadata        notes
  #> 6        alternative /metadata        notes

  ^{Created on 2023-03-28 with reprex v2.0.2} (Those are actually not needed, and won't be done here)

• [x] Demonstration of deposits_metadata_template() workflow.

[mpadge]

The above commit adds most of the vignette, with the above TODO extended to include remaining tasks.