docs: add a Translations page and cover the overlay countdown inheritance

[enoch85]

Addresses the translation and overlay parts of the drift report in #148.

What was added

Doc	Change	Upstream
docs/Translations.md (new)	Own sidebar section. How to pick a Display language in Settings -> General, that the choice lives in the browser and untranslated text falls back to English, the 13 languages, that only the web UI is translated, and an invitation to translate on Weblate	#3486, #3488, #3490
sidebars.js	New Translations category (the only non-docs file touched, required by the sidebar entry)	-
docs/Configuration.md	Display language row in the General table, pointing at the new page	#3486
docs/Overlays.md	New "Media that gets deleted along with a collection": what a deleting collection also draws on (descendants, and a season or show left empty), that those items are never added to the collection, which actions and collection types this covers, the title card template needed for episodes, and holding overlays when the media server cannot be read	#3471
docs/Overlays.md	Template kind follows the item drawn on, each kind keeps its own default (a Default badge in both lists is expected), the editor shows the kind and it is fixed after creation, deleting a collection restores its overlays first	#3471, #3475

Deduplicated while in there: poster/title card mode, the template fallback and the Run Now forced pass were each stated twice or three times, now once each.

Not in this PR

Drift item	Status
Webhook media items carrying type, title and providerIds (#3464)	Still undocumented. docs/Notifications.md shows mediaServerId and requestedBy only, so this needs a follow-up
The calendar naming the season that leaves (#3472)	Still undocumented, docs/Calendar.md does not mention seasons
Test Media no longer re-reading the whole Tracearr history (#3483)	Performance fix with no documented behavior to correct
overlay-template.ts contract change	Internal helper (overlayModeForType), no API surface, so no API.md or OpenAPI change