[docs]: document instance id

#### Problem

The concept of an instance-id is core to cloud-init's behavior, yet no user-consumable definition exists. The idea of an instance-id is exposed directly in some of cloud-init's user-interfaces[1][2][3].

The content that best defines instance-id is [this page](https://cloudinit.readthedocs.io/en/latest/explanation/first_boot.html). This page has several problems:

- not discoverable as the definition of instance id
- unwieldy and context-heavy title "First boot determination" is not user-friendly
- much of this content explains implementation details which do not concern the user

#### Proposed fix

1. The idea of an instance-id should be briefly introduced in the [core tutorial](https://docs.cloud-init.io/en/latest/tutorial/qemu.html#define-our-metadata)
2. We should have a page which gives a discoverable user-friendly description of instance-id.
3. These pages should include reference links to this page[4].

#### References
[1] Every module definition has a [module frequency](https://docs.cloud-init.io/en/latest/reference/modules.html), which is often once-per-instance, which has to do with instance-id.
[2] https://docs.cloud-init.io/en/latest/reference/modules.html#scripts-per-instance
[3] https://docs.cloud-init.io/en/latest/howto/module_run_frequency.html
[4] referencing pages:
https://docs.cloud-init.io/en/latest/tutorial/qemu.html
https://docs.cloud-init.io/en/latest/reference/datasources/configdrive.html#instance-id
https://docs.cloud-init.io/en/latest/development/internal_files.html#instance
https://docs.cloud-init.io/en/latest/reference/datasources/vmware.html
https://docs.cloud-init.io/en/latest/reference/datasources/fallback.html#fallback-no-datasource
https://docs.cloud-init.io/en/latest/reference/datasources/ec2.html
https://docs.cloud-init.io/en/latest/reference/datasources/wsl.html#vendor-and-metadata
https://docs.cloud-init.io/en/latest/explanation/format.html#example-user-data-script
https://docs.cloud-init.io/en/latest/reference/datasources/nocloud.html#example-creating-a-disk