Clarify the Jira Server credential steps

What does this MR do?

Following the discussion in !118655 (comment 1378222347), this MR does the following:

1. Clarifies that an existing Jira Server user can be used to configure the Jira integration in GitLab
2. Reiterates in the top section that once a Jira Server user has been selected that the Jira integration needs to be configured (and links off to that)
3. Links to the Atlassian docs on how to create a user. I put this in so its clear to readers that they must click Create user rather than Invite user.
4. Links the permission section back to the groups section
5. Changes the Email address and Username requirements to match up with my observations in !118655 (comment 1377531476)
   • The Email address did not need to be valid for me. I've changed this to recommend to use a valid email address instead.
   • The Username was not based on the email address and this was not imposed on me by Jira. Since our docs is walking the reader through how to setup a Jira user with the username gitlab, let's make that explicit here.
   • The Password field can be set and applied at the same time a Jira user is created. The docs currently imply you need to come back after the user is created to set the password which is not true.

Related issues

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