AP-362--Fix-incorrect-content-on-the-Register-and-manage-your-service-page

Why

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
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build, check links and run locally using nginx by running ./preview-with-nginx.sh
ensure page http://localhost/before-integrating/register-and-manage-your-service/ makes sense

no changelog entry required for fix

[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