[REF] crm: removed references to base_stage in crm_lead / crm_claim + various pre-cleaning before removing state.
[DOC] crm: added basic doc (index.rst + changelog)
[DOC] crm: improved changelod.
[REF] crm, crm_claim, crm_helpdek: removed inheritance from base_stage and first cleaning of stage/stage management.

Most of state update methods are removed (like case_cancel and case_reset). This is because we are moving towards removing state and the related methods where stage is already present. Also removed some dead tests about case update.

Some methods are added back from base_stage like case_escalate, to remove the inheritance.

[CLEAN] crm, crm_claim, crm_helpdesk: misc cleaning in modified files/methods about spaces and unused variables

[DOC] added doc directory + changelog