docs(addon-development): Summary sections and verified corrections

[eduralph]

Follow-up to #994. Regenerated from the docs-as-code source; the vault stays the single source of truth and this tree is deterministic output.

What changed

Every page now opens with a real Summary — what it covers, in what order, and when to come back to it — instead of a one-line stub.

Corrections, each checked against this repository and gramps on maintenance/gramps60:

Page	Change	Evidence
03-addon-kinds	14 → 16 registration kinds	REPORT(0)…CITE(15) in gramps/gen/plug/_pluginreg.py; QUICKREPORT is a deprecated alias of QUICKVIEW, not a separate kind
16-guidelines	addon-root __init__.py: MUST NOT → SHOULD NOT, with a MAY carve-out	Seven addons here ship one and work — DynamicWeb, ExcludeSubtreeFilter, GrampyScript, PlaceCleanup, PostgreSQLEnhanced, Query, Sqlite (six are empty files)
04-fundamentals, 16-guidelines	get_addon_translator must be guarded	It raises ValueError when no compiled catalog exists and no configured language starts with en/C
09-troubleshoot	new entry: CustomFilters is None under test	reload_custom_filters() rebinds the module global, so importing the name first captures None permanently
12-packaging	LANGUAGE='en_US.UTF-8' dropped from the make.py recipe	make.py on this branch never reads it; adds the as-needed and manifest-check targets
07-testing	tests/ package asymmetry re-argued	Old rationale didn't hold — see below

On the __init__.py downgrade

The previous rule cited Mantis 12691 in support. That bug is a different trap — from <Addon> import <Addon> binding the submodule instead of the class, caused by the addon dir's parent being on sys.path — which a namespace package hits identically. __init__.py neither causes nor cures it, so the citation is withdrawn.

What the file actually costs is consistency across contexts: run from the repo root, as tests are, the same module is also reachable as <Addon>.<module>, giving two module objects and two copies of every module-level global. PluginManager.import_plugin inserts the addon's own directory at sys.path[0] and does a top-level __import__, so the root __init__.py never runs on the load path at all.

A rule that seven shipping addons violate without producing a defect doesn't meet this page's own bar for MUST. 17-roadmap records the open question of whether CI should flag it — advisory at most, since a gate would start red.

Notes

• README.md and _media/ are unchanged; the diff is exactly the 17 pages.
• The export is deterministic — re-running it against this branch leaves an empty git diff.
• A matching PR targets maintenance/gramps61.
• Draft until reviewed.