docs: add info about ODK Entities (#239)

* Began documenting work on Entities-based FMTM work

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* docs: extra info on entity creation

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docs/about/odk-entities.md

@@ -1 +1,71 @@
 # ODK Entities
+
+Entities are a concept introduced into ODK around 2022. The basic goal
+of them is to allow updating of data using ODK, specifically by revisiting
+specific things (features, people, whatever) and adding new data attached
+to the same entity (data people refer to this as "longitudinal data"). For
+example, revisiting the same patient in a clinical study, such that the
+patient ID is constant and the new data from each visit is referenced to
+the same person. For field mapping, the use-case is obvious: visit a
+previously digitized building and add data from in-person observation or
+surveying the people in it.
+
+As of time of writing (March 2024), Entities in ODK are working and more
+or less implemented, but not yet in wide use or well debugged.
+
+# Use of Entities with FMTM
+
+In FMTM, we want to:
+
+- Be aware of features that have already been mapped
+  - the ODK map views don't yet support styling features differently
+    based on attributes like "already_field_mapped", so we're actually
+    working on a navigation app within FMTM external to ODK, but we still
+    hope that ODK Collect may support styling in the future, which will
+    almost certainly be based on Entity attributes.
+- Pre-fill questions in the form when features already have data attached
+  to them (for example, if a building already has a "name" tag, the field
+  mapper should see this pre-filled in the form, to be overwritted if wrong
+  but otherwise swiped past).
+
+The ODK core development team has strongly suggested that the FMTM team
+use the Entities to achieve the above goals.
+
+## Workflow Using Entities
+
+The basic workflow would probably resemble:
+
+- Create an Entity registration form.
+  - In standard ODK settings, this simply means adding an `entities`
+    tab to the XLSForm (as per
+    [the example Entities form created by the ODK team](https://docs.google.com/spreadsheets/d/1xboXBJhIUlhs0wlblCxcQ3DB5Ubpx2AxLDuaXh_JYyw/edit#gid=2050654322).
+    This seems to create what ODK refers to as a Dataset (in developer-facing documenation only;
+    they avoid this word in user-facing documentation).
+- Upload the Entities via the API:
+  - There is currenltly no way to bulk upload Entities to a Dataset via the API.
+  - Instead we must upload each Entity individually, including a geometry field
+    in JavaRosa geometry format.
+  - Until time of writing, FMTM creates geography attachments in GeoJSON,
+    which does not work for Entities, which require a CSV file with a
+    `geography` column (which must be in JavaRosa format, which no self-respecting
+    GIS utility can export).
+- Create a form for the data collection, referencing the `{dataset_name}.csv` for
+  the `select_one_from_file` field.
+- Dynamically insert the `task_id` field on the `choices` sheet, with `name` and
+  `label` set to the task id (for filtering later).
+- Create a field `task_id` in the survey, prior to the `select_one_from_file` field.
+- Set `choice_filter` column to task_id=${task_id}, which links the selected task
+  ID in the survey with the task_id field in the Entities (used for filtering).
+- Load the form via intent, with `task_id` and `entity_id` fields in query params.
+- On form completion we can update the Entity label and fields.
+  - Set the field `status` to COMPLETE, IN_PROGRESS, INVALID.
+  - Update the entity label to be prefixed with ✅ ❌ for a nicer visual cue.
+    <https://docs.getodk.org/central-entities/#building-a-form-that-updates-an-entity>
+
+# Resources
+
+- [Here's the introductory page from ODK discussing the rationale for and use of Entities](https://docs.getodk.org/central-entities/). It has some example forms you can use to get started figuring out how to use Entities.
+- [Here's the specification for Entities in the XForms language](https://getodk.github.io/xforms-spec/entities).
+- [Here's the ODK Central API documentation for dealing with Entities](https://docs.getodk.org/central-api-entity-management/)
+- [Here's the ODK Central API documentation for dealing with the "Datasets" they are part of](https://docs.getodk.org/central-api-form-management/#related-datasets)
+- [Here's a discussion of how you can attach a draft form to an entity "dataset"](https://docs.getodk.org/central-api-form-management/#linking-a-dataset-to-a-draft-form-attachment)