docs: add how-to for merging legacy library exports into a new library archive

[irfanuddinahmad]

Summary

• Adds source/educators/how-tos/course_development/merge_legacy_library_exports.rst — a step-by-step guide for merging one or more legacy (V1) library exports into a single new-library backup archive by hand, without an automated migration tool.
• Wires the new page into the course_development how-tos toctree (source/educators/navigation/content_creation_management.rst), alongside the existing migrate_legacy_libraries.rst one-click migration guide.
• Cross-links the new page from source/educators/references/library_archive_format.rst's seealso block.

This content was originally proposed in openedx/frontend-app-authoring#3087, but per review feedback it belongs here instead, next to the existing legacy-library migration docs. That PR has been closed in favor of this one.

Closes openedx/frontend-app-authoring#2877

Test plan

• Built the docs locally with the repo's own CI check (sphinx-build -T -W --keep-going, warnings as errors) — clean build, and all :ref: cross-references resolve.
• Spot-checked the rendered HTML output for the new page, including heading hierarchy, tables, and code blocks.
• Verified the live Read the Docs PR preview: https://docsopenedxorg--1519.org.readthedocs.build/en/1519/educators/how-tos/course_development/merge_legacy_library_exports.html

[openedx-webhooks]

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