docs(col-group): fix body/edit template docs; selectable setter

[damyanpetev]

IgxColumnGroupComponent has hidden and non-functional setter overrides for bodyTemplate and inlineEditorTemplate, but had the getters left public, which is pretty useless. Hidden those as well as the selectable setter, since it only works as getter on the group.

Additional information (check all that apply):

• Bug fix
• New functionality
• Documentation
• Demos
• CI/CD

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code (test guidelines)
• This PR includes API docs for newly added methods/properties (api docs guidelines)
• This PR includes feature/README.MD updates for the feature docs
• This PR includes general feature table updates in the root README.MD
• This PR includes CHANGELOG.MD updates for newly added functionality
• This PR contains breaking changes
• This PR includes ng update migrations for the breaking changes (migrations guidelines)
• This PR includes behavioral changes and the feature specification has been updated with them