docs(admin): Clarify DB conversion steps and command usage

[joshtrichards]

This clarifies the conversion process significantly, based on a mixture of help forum interactions and my own testing. Namely:

• Make it clearer you should set-up your target database first
• Link to respective db configuration sections so reader sees how to add new target db + credentials
• List type parameters supported
• Clarify process/steps
• New call-outs where appropriate
• Add that mysql.utf8mb4 parameter should get set to true after converting (if targeting MariaDB/MySQL)
• General clarifications and language adjustments

☑️ Resolves

Also:

• Closes Document valid strings for occ:convert-type database type #7341

Follow-up note: I'll add language in a follow-on PR for UNIX socket usage when that gets fixed in server (nextcloud/server#31998) - i.e. when I finish nextcloud/server#39242 probably.

🖼️ Screenshots

[joshtrichards]

/backport to stable28

[joshtrichards]

/backport to stable27

[backportbot]

The backport to stable28 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable28
git pull origin stable28

# Create the new backport branch
git checkout -b backport/11584/stable28

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 2874107d a2fa5857 58f5f572 23e6fb02 ee7c2ba0 5f4ba48e 16e1b357 c66db26c e900d173 fd84d8cb

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/11584/stable28

Error: Failed to cherry pick commits: error: no cherry-pick or revert in progress
fatal: cherry-pick failed

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[backportbot]

The backport to stable27 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable27
git pull origin stable27

# Create the new backport branch
git checkout -b backport/11584/stable27

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 2874107d a2fa5857 58f5f572 23e6fb02 ee7c2ba0 5f4ba48e 16e1b357 c66db26c e900d173 fd84d8cb

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/11584/stable27

Error: Failed to cherry pick commits: error: no cherry-pick or revert in progress
fatal: cherry-pick failed

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.