Document single-channel guest licensing changes

[marianunez]

Summary

• update guest account documentation to explain single-channel guest billing, DM/GM handling, and admin-only overage warnings
• add reporting and Edition and License documentation for the Single-channel Guests metric and related visibility
• align subscription, FAQ, and plan-comparison docs with the new seat-count model

Test plan

• Reviewed changed doc pages for consistency and stale guest billing language
• Checked edited files for linter issues with Cursor diagnostics
• Build docs locally with Sphinx (gmake html or pipenv run sphinx-build -M html source build -j auto) once the required tools are available

[github-actions]

Newest code from mattermost has been published to preview environment for Git SHA 2307328

[marianunez]

@esethna created this PR with Claude based on the doc review feedback from the feaure PR here: mattermost/mattermost#35451 (comment)

Not sure what else needs to be updated for this feature.

[github-actions]

Newest code from mattermost has been published to preview environment for Git SHA 9cbc7df

[Combs7th]

@esethna - This docs PR should cover the following:

mattermost/mattermost#35451 (Add single-channel guest tracking and reporting)
mattermost/mattermost#35517 (Add single-channel guests filter and channel count column to System Console Users)
mattermost/mattermost#35846 (Add single-channel guest count to support packet stats)