Clean up developer guide wording and align Indonesian links

[yukuku]

The developer guides contain leftover wording about original or historical documentation, screenshot age disclaimers, and missing links in the Indonesian publishing paragraph. This cleanup makes the guides describe the current formats directly and keeps the English and Indonesian references aligned.

Implementation

• Replaces the Indonesian reference to lyrics preserved from the original guide with a description of the song-book format examples.

• Removes screenshot age disclaimers from the verse-opening guide and both language versions of the song guide.

• Gives the Indonesian publishing paragraph the same song authoring specification, Songs module guide, and portable-songs design links as the English paragraph.

• Describes each publishing reference by its subject, removing the historical-document caveat in both languages.

• States the YET Indonesian locale requirement directly, using in rather than id, without historical wording.

• Removes the reading-plan guide's instruction to contact support to edit, replace, or delete a submitted plan.

Technical details

• Changes only docs/developer/songs.md, docs/developer/opening-verses.md, docs/developer/yet.md, and docs/developer/reading-plans.md.
• Preserves code blocks, screenshots, image captions, and technical format requirements.
• Follow-up to the merged documentation migration in Migrate developer guides and modernize the Kotlin extension sample #309; its verbatim import and editing commits remain intact.

Validation

• Read and rendered all six developer guides and the extension sample README.
• Scanned English and Indonesian wording for original/earlier documentation references and screenshot age statements; none remains on this branch.
• Checked 46 relative links, anchors, and image references; all targets exist.
• Confirmed both publishing paragraphs retain the same three reference links.
• Confirmed code blocks and image captions are unchanged by the final sweep.
• Confirmed the submitted-plan email instruction no longer appears in any developer guide.
• git diff --check passed. Gradle builds were not rerun for prose-only edits.

[github-actions]

📱 Preview builds

Signed release builds of 220217f — version 5.0.0-dev.124 (23958460) — from this run.

Flavor	Application ID	APK
Alkitab	yuku.alkitab	Download (7.6 MB)
Quick Bible	yuku.alkitab.kjv	Download (7.5 MB)
Sabda Alkitab	org.sabda.alkitab	Download (7.6 MB)

Or open https://f8ab1a64-alkitab-pr.yukuku.workers.dev on an Android device.

These share their application IDs and signature with the Play Store builds, so installing one replaces the corresponding installed app (data is kept).

This comment tracks the latest build for this PR; earlier builds keep their own URLs.