Merge branch 'el-update-fe-guide-vue-ffs' into 'master'

Update doc on how to use glFeatureFlagsMixin

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/124386



Merged-by: Phillip Wells <pwells@gitlab.com>
Approved-by: Phillip Wells <pwells@gitlab.com>
Co-authored-by: Eugie Limpin <elimpin@gitlab.com>