docs: document period type label and displayLabel [DHIS2-20845]

[karolinelien]

Documents the label / displayLabel support on PeriodType added in 2.43, following up on dhis2/dhis2-releases#261.

Adds a Period types subsection to the Web API introduction chapter (src/developer/web-api/overview.md), inside the existing Date and period format section and just after the period-format table, since that is where period types are already described. Covers GET /api/periodTypes with a property table marking label as the only writable field, plus a Custom period type labels subsection with the PUT request, the success response, the verify response, and clearing a label with an empty string.

Three details were checked against dhis2-core rather than taken from the release note, and differ from it:

• The release note's example body includes all fields (isoDuration, frequencyOrder, and so on). PeriodTypeController.putPeriodType only reads getName() and getLabel(), so everything else is silently ignored. The docs show name + label only.
• An unknown name returns 400 Bad Request with the message "<name> does not exist." — DefaultPeriodService.updatePeriodTypeLabel throws IllegalArgumentException, which CrudControllerAdvice maps to badRequest. The status was not stated anywhere.
• displayLabel is null, not displayName, when no label is set — PeriodType.getDisplayLabel() returns label directly (with a TODO for the real implementation). The section therefore tells clients to render displayLabel and fall back to displayName, so apps adopting displayLabel now do not show blanks.

Also documents the ALL authority requirement (@RequiresAuthority(anyOf = ALL)).

Targets master only. If the 2.43 docs should carry it as well, that is a cherry-pick onto the 2.43 branch.

AI Assisted

[maikelarabori]

Looks good. Just one minor comment.

Thx!

[maikelarabori]

You can also do:

{
  "name": "FinancialFeb",
  "label": null
}