Add documentation for the TLS Vault feature

[fabi200123]

This PR adds the documentation for the TLS Vault feature of Sunbeam.

NOTE: This PR shouldn't be merged until the snap-openstack PR is merged, because the documentation might be changed accordingly to the spec for this feature.

[hemanthnakkina]

Some review comments on the first pass

https://canonical-openstack.readthedocs-hosted.com/en/latest/how-to/misc/implement-tls-using-a-third-party-ca/ also needs to be updated.

I will talk with documentation team if there is any restructuring required given TLS has support with CA and Vault now.

[hemanthnakkina]

There are some other places like https://canonical-openstack.readthedocs-hosted.com/en/latest/reference/manifest-file-reference/, https://canonical-openstack.readthedocs-hosted.com/en/latest/explanation/service-endpoint-encryption/, https://canonical-openstack.readthedocs-hosted.com/en/latest/reference/interactive-configuration-prompts/ that needs update

[hemanthnakkina]

@fabi200123 CI Spell check failures, just cross check if those words are used in TLS CA in capitalised form or you can add them in ignore list

[skoech]

Hi @fabi200123 and @hemanthnakkina, I've left some suggestions and nitpicks.

[hemanthnakkina]

I am thinking over the lines like TLS enablement under Features and then subpages for CA and Vault.

This page also requires update (The content should be generic for both TLS CA and Vault except the output for outstanding certificates differ).

https://canonical-openstack.readthedocs-hosted.com/en/latest/how-to/misc/implement-tls-using-a-third-party-ca/

[wolsen]

LGTM