Document translation template generation

[ThomasConway01]

Adds a dedicated guide for the translation template generator and links it from the gettext workflow.

It covers configuring PO and CSV output, Command Palette access, the kinds of strings that are extracted, Auto Translate Mode, translation contexts, and EditorTranslationParserPlugin._customize_strings() for custom processing.

Closes #12219.

Validation:

• codespell passed on the three edited files.
• git diff --check passed.
• A complete local Sphinx build was started after installing the documented dependencies, but did not complete within the available command time. CI can run the full documentation build.

[Cykyrios]

The changes are alright, but wording and general flow can be improved. However, before getting into that, I would just like to confirm whether this PR used AI? It very much looks to me like an AI agent was used, so you should mention it if that's the case, as per the Contributing policy.