Added help pages

gms-ui/src/components/TopMenu.vue

gms/src/main/java/it/inaf/ia2/gms/authn/SecurityConfig.java

gms/src/main/resources/static/help/help-admin.html

gms/src/main/resources/static/help/help-pi.html

...in/resources/static/help/img/gms-add-collaborator-btn.jpg

.../resources/static/help/img/gms-add-collaborator-modal.jpg

gms/src/main/resources/static/help/img/gms-add-group-btn.jpg

...src/main/resources/static/help/img/gms-add-member-btn.jpg

...main/resources/static/help/img/gms-add-permission-btn.jpg

...n/resources/static/help/img/gms-admin-add-group-modal.jpg

.../resources/static/help/img/gms-admin-add-member-modal.jpg

...ources/static/help/img/gms-admin-add-permission-modal.jpg

gms/src/main/resources/static/help/img/gms-admin-groups.jpg

...src/main/resources/static/help/img/gms-generic-search.jpg

gms/src/main/resources/static/help/img/gms-pi-groups.jpg

gms/src/main/resources/static/help/img/pencil-icon.jpg

gms/src/main/resources/static/help/img/trash-icon.jpg

gms/src/main/resources/static/help/index.html