Improved Start here section

[ainoc]

Proposed changes

Improved Start here section: https://csc-guide-preview.rahtiapp.fi/origin/ac-sdaccess-and-login-pages/data/sensitive-data/sd-access/

Checklist before requesting a review

• [ ] I have followed the instructions in the Contributing and Styleguide documents.
• [ ] My pull request passes all tests.
• [ ] I have included a link to the appropriate preview page (select your branch from the list).

[rkronberg]

Hi! The tests are not passing as there seems to be a few broken links, as well as incorrectly formatted internal links (https://docs.csc.fi/...). Instructions how check why the tests did not pass: https://github.com/CSCfi/csc-user-guide/blob/master/FAQ.md#my-pr-did-not-pass-the-tests-what-to-do