Currently, res.partner has:
1. `website_id` field hardcoded in website module
2. `website_published` field from website `website.published.mixin` in
website_partner module.
At the end, the res.partner model has all the field of the mixin
`website.published.multi.mixin` without having the mixin.
As a result, we have all the multi-website fields but we don't have access to
the mixins methods & computes.
Now, we use the `website.published.multi.mixin` on res.partner in website
module even if `website_published` is not needed if website_partner is not
installed.
+ add tooltip to `website` field to prevent warning since 2 fields would have
the same label
closesodoo/odoo#29711