Geo - Alter the steps for creating an external tracking database

What does this MR do?

1. The current instructions imply that the user already has a database to use for their tracking database. Borrowing the instructions from steps 1-2 in https://docs.gitlab.com/ee/administration/postgresql/external.html, let's make this more explicit here so there is no confusion.
2. I've added If you are **not** using a cloud managed PostgreSQL database to step 3 - this currently leads to confusion for the reader if this does not apply to them. Not sure if a note would be more appropriate here - let me know.
3. Fixed spelling mistake in one of the links.
4. When reconfiguring the secondary node after entering in the tracking database details, GitLab will automatically migrate the tracking database. This makes me wonder if the gitlab-rake geo:db:create and gitlab-rake geo:db:migrate commands are actually worth mentioning. I feel the gitlab-rake geo:db:migrate comes in handy if you ever need to run it manually (and no harm in running it again after the reconfigure), but unsure about the gitlab-rake geo:db:create command - this could generate an error if the database already exists or the database user is not a superuser. cc @fzimmer - could you give some direction here or help me find someone who can?

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