Docs: Add information about support for MCP Clients to Foundational & Custom Flows

• Start this issue's title with Docs: or Docs feedback:.

Problem to solve

MCP clients are supported as part of any flows, not only Software development flow. It needs to be updated in the documentation.

Further details

N/A

Proposal

Update documentation to match the existing technical setup.

Who can address the issue

Any GitLab Team Members

Other links/references

N/A