Fixes for the SLE 16.0 release

[lslezak]

Update the documentation to match the 16.0 release

Some Agama features were added in 16.1 and are not present in 16.0 or behave differently. Update the documentation to match the 16.0 behavior.

• The self update feature is available only in 16.0/QU, all later QUs disable the self update because we do not provide any self-update repository for them. However, it is still possible to use a custom self update repository if needed.
• The inst.remote, inst.solver_testcase and inst.systemd_boot_preview boot options are not supported in 16.0.
• Fixed JSON schema URL location for SLE-16.0 (16.1 uses a different path)
• The 16.0 JSON schema definition does not allow using the $schema key, it needs to be removed before using the profile otherwise Agama reports an invalid profile. In 16.1 the key is allowed but still ignored by the installer. Because the 16.0 installer fails when the key is present I changed the <note> tag to <warning>.

Feel free to improve the changes or the texts, I'm not a documentation expert. 😃

PR creator: Are there any relevant issues/feature requests?

• SME review: scope check for 16.0 Agama docs #764

PR reviewer: Checklist for editorial review

Apart from the usual checks, please double-check also the following:

• do any new files fulfill the naming conventions specified in https://github.com/SUSE/doc-modular/blob/main/templates/README.md?
• article title/structure title: does it have the required length and does it use sentence style capitalization?
• revhistory: is it up-to-date and does it follow the guidelines specified in https://documentation.suse.com/style/current/html/style-guide-db/sec-smartdocs.html#sec-revhistory?
• metadata: does it follow the guidelines specified in https://documentation.suse.com/style/current/html/style-guide-db/sec-smartdocs.html#sec-taglist-smartdocs?