Add multi-tenancy guide (doc/site) (#2322)

Signed-off-by: Avgustin Marinov <Avgustin.Marinov@bosch.com>

site/.gitignore

@@ -8,6 +8,7 @@ node_modules
 package.json
 package-lock.json
 # themes
+themes
 themes/hugo-material-docs
 # hugo
 public

site/content/guides/multitenancy.md

@@ -0,0 +1,46 @@
+---
+title: Multi-tenancy
+parent: Guides
+weight: 32
+---
+
+hawkBit is able to support with multiple tenants with some constraints. This guide provides insights in the basic concepts and how to setup your multiple tenants.
+<!--more-->
+
+## Big picture
+
+Multi-tenancy is a core concept of hawkBit. It is supported by providing hawkBit authentication system supporting tenants as well as by using EclipseLink and Hibernate native multi-tenant support.
+
+The tenant and user management, however, is only static. This means you shall configure all tenants and users into spring configurations. 
+
+The tenant and user management could be extended to be more flexible - e.g. dynamic management, OAuth and so on. This however is left for integrators for specific use cases.
+
+## Constraints
+* static tenant and user management
+* configuration only via TenantAwareUserProperties#user configuration (i.e. hawkbit.security.user properties), hence
+* username is unique for the whole system - there could not be same username for different tenants
+* in order to create dynamically tenants when user is logged in you shall set hawkbit.server.repository.implicitTenantCreateAllowed=true, 
+
+## Example
+
+```properties
+# allow to auto/implicit create DEFAULT tenant (on mgmt api call)
+hawkbit.server.repository.implicitTenantCreateAllowed=true
+# setup foo tenant with two users john and smith_on_foo
+hawkbit.security.user.john.tenant=foo
+hawkbit.security.user.john.password={noop}john_password
+hawkbit.security.user.john.roles=TENANT_ADMIN
+hawkbit.security.user.smith_on_foo.tenant=foo
+hawkbit.security.user.smith_on_foo.password={noop}smith_password
+hawkbit.security.user.smith_on_foo.roles=TENANT_ADMIN
+# setup bar tenant with single user smith_on_bar
+hawkbit.security.user.smith_on_bar.tenant=bar
+hawkbit.security.user.smith_on_bar.password={noop}smith_password
+hawkbit.security.user.smith_on_bar.roles=TENANT_ADMIN
+```
+This example creates two tenants foo and bar:
+* foo has two users john and smith_on_foo
+  * john/john_password
+  * smith_on_foo/smith_password
+* bar has single user smith_on_bar (it is again smith but since the user smith user could not be used for both tenants foo we need to use different username)
+  * smith_on_bar/smith_password

site/layouts/partials/header.html

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