Improve inline embedding URL handling

What does this MR do and why?

We currently allow the user to embed GitLab Observability UI in the markdown editor by expanding any observe.gitlab.com links. We want to now remove support for observe.gitlab.com and allow inline embedding only through GitLab URL, e.g. https://gitlab.com/groups/gitlab-org/opstrace/-/observability/explore.

We also want to restrict inline embedding to the explore and goto path, as we don't want the user to be able to inline embed any Observability URL.

As part of this change, I've also included some changes that tackles a reported security vulnerability (https://gitlab.com/gitlab-org/security/gitlab/-/issues/821+).

• Support embedding GOUI by sharing GitLab URL (gitlab-org/opstrace/opstrace-ui#246 - closed)
• Match sharable GOUI urls only (gitlab-org/opstrace/opstrace-ui#153 - closed)
• https://gitlab.com/gitlab-org/security/gitlab/-/issues/821+

Screenshots or screen recordings

How to set up and validate locally

• Enable Observability feature flag Feature.enable(:observability_group_tab)
• Open a markdown editor ( e.g. issue description or comment )
• Insert an observability link e.g. https://local.gitlab.com/groups/flightjs/-/observability/explore?observability_path=/explore
• See the observability iframe being rendered ( might show a "Not Found" warning, depending on how local GOUI is configured )

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.