[ADD] documentation for OpenUpgrade manager. how to create reference database.

[legalsylvain]

replace old wiki. see https://github.com/grap/openupgrade_wiki_backup/blob/master/How-to-create-a-reference-database.md

What I did.

• copy paste old wiki page into OpenUpgrade documentation
• split into a rst part and a sh part
• add an extra comment to mention that we have to make some retries, if some odoo modules requires extra python lib. see for exemple : asn1crypto remarks.
• I changed shell command, using cat & pipe. it avoid to have to copy paste orm commands.

blocking point
I have a blocking problem regarding force_storage. here is the log of the error, generated by l10n_de module in V17.

  File "/home/sylvain/grap_dev/grap-odoo-env-17.0/src/odoo/odoo/addons/base/models/ir_attachment.py", line 81, in force_storage
    ]))._migrate()
  File "/home/sylvain/grap_dev/grap-odoo-env-17.0/src/odoo/odoo/addons/base/models/ir_attachment.py", line 89, in _migrate
    attach.write({'raw': attach.raw, 'mimetype': attach.mimetype})
  File "/home/sylvain/grap_dev/grap-odoo-env-17.0/src/odoo/addons/l10n_de/models/ir_attachment.py", line 31, in write
    self._except_audit_trail()
  File "/home/sylvain/grap_dev/grap-odoo-env-17.0/src/odoo/addons/l10n_de/models/ir_attachment.py", line 27, in _except_audit_trail
    raise UserError(_("You cannot remove parts of the audit trail."))
odoo.exceptions.UserError: You cannot remove parts of the audit trail.

-> What is the best way to move forward ? I'd like to avoid to have to write / maintain odoo patches in the database creation process.
-> is this step mandatory ?

@OCA/openupgrade-maintainers : thanks for your help here !

[StefanRijnhart]

You could consider moving towards an Odoo style backup package that includes the filestore. Alternatively, you can just comment out this code temporarily while you move the filestore into the database: https://github.com/odoo/odoo/blob/17.0/addons/l10n_de/models/ir_attachment.py#L29-L32

[legalsylvain]

Thanks !