Make start of week configurable via configuration file/command line

For provisioning purposes, it would be great to be able to configure the start of the week via the configuration file. In #3073 I could only see that the option was added in the UI.

What did you do?
Try to configure the start of week via the configuration file

What did you expect to see?
A configurable option described in the documentation

What did you see instead? Under which circumstances?
No configurable option. Under any circumstances 😃

Environment

• System information:

Linux 5.15.0-118-generic x86_64

• Alertmanager version:

alertmanager, version 0.27.0 (branch: HEAD, revision: 0aa3c2aad14cff039931923ab16b26b7481783b5)
  build user:       root@22cd11f671e9
  build date:       20240228-11:51:20
  go version:       go1.21.7
  platform:         linux/amd64
  tags:             netgo

• Prometheus version:

n/a

• Alertmanager configuration file:

n/a

• Prometheus configuration file:

n/a

• Logs:

n/a

Review the UI option introduced in issue #3073 and the Alertmanager configuration documentation linked in the issue. Trace how configuration-file and command-line settings are defined and loaded, then verify that the start-of-week value can be provisioned through the requested interfaces and is documented.