AP-362 fix for sse admin

[pauldougan]

AP-362

Why

Corrections to documentation relating to self service

What

- remove references to the registration page, this is no longer used in integration
- link directly to the self service tool to register and create a client
- add points 2 and 3 to cover confirmation of the email address and mobile number that are not covered by the table
- move all external and internal links into partials/_links.erb
- remove redundant paragraph duplicating content about the ID token signing algorithm
- remove redundant update your services details section that was using the old form, all changes should be via the self service tool

Technical writer support

Do you need a tech writer's support, for example to review your PR? YES

How to review

• /preview-with-nginx.sh
• open http://localhost/before-integrating/register-and-manage-your-service/ and review in context

ensure that the new content is consistent and links to the self service way in a way we believer will make sense to users

Changelog

Yes

Confirm

• [x] I have checked if any docs change here also requires updates to other repositories (ADRs / RFCs, README.md, Team Manual, elsewhere in these docs)
• [x] Where there is any overlap I have updated or opened a PR for corresponding changes

[pauldougan]

I tested this locally and previewed the content and it works as intended