docs: tag mutability adr

[jesperhodge]

This is a draft PR, generated with the help of AI, purely for basis of discussion. Not ready to be merged.

We had a chat around mutability of taxonomy tags: https://openedx.slack.com/archives/C09A1BN1CSE/p1771871556363759

I used AI to generate this ADR, as a starting point to discuss further inside this PR. It's just designed to summarize the problem and allows us to workshop it.
I reviewed the AI output and generally find it accurate.

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[bradenmacdonald]

I think this is a good summary of our conversation.

As for how to handle external_id, I think a compromise might be:

• The taxonomy editor UI displays external IDs, but doesn't allow changing them. It may allow specifying them when creating a tag, and/or generate one based on the value if none is specified.
• In the rare case where an external ID needs to be changed, that can be easily done by an administrator using the Django admin UI.

[jesperhodge]

I think this is a good summary of our conversation.

As for how to handle external_id, I think a compromise might be:

• The taxonomy editor UI displays external IDs, but doesn't allow changing them. It may allow specifying them when creating a tag, and/or generate one based on the value if none is specified.
• In the rare case where an external ID needs to be changed, that can be easily done by an administrator using the Django admin UI.

Sounds good to me. Do you agree with this solution @mgwozdz-unicon ? In that case I'll put that in the ADR decision block and mark this as ready for review