Add documentation for base concepts

docs/admin/01_core_concepts.rst

+The AlekSIS core
+================
+
+The AlekSIS core provides functionality and data models as a base for
+all apps.
+
+.. _sec:SchoolTerms:
+
+The concept of school terms
+---------------------------
+
+In AlekSIS, mostly everything is based on school terms. A school term is
+a defined time range which can be used to link data to it. Typically,
+such data are learning groups, timetables or class register records.
+Although their name suggests it, school terms don’t have to be a half or
+a full year. They should depend on the way how you organise data in your
+institution.
+
+For example, if you issue reports at the end of every half year, a half
+year would be a good time range for your school terms because the class
+register statistics are evaluated for school terms.
+
+Anyway, you should create a school term before you start to import or
+create other data entries like persons or groups.
+
+Manage school terms
+~~~~~~~~~~~~~~~~~~~
+
+You can manage the school terms if you login with your admin account and
+open the menu entry ``Admin → School terms``. There you can find a list
+of all school terms and buttons to create, edit or delete school terms.
+Please be aware that there can be only one school term at time and each
+school term needs a unique name.
+
+.. _sec:Persons:
+
+The concept of persons
+----------------------
+
+The person model is designed to save all the data of students, teachers,
+guardians and any other persons of the school society. It tracks
+information like the following:
+
+-  Full name
+-  Short name
+-  Sex
+-  Date of birth
+-  Contact details (phone numbers, email)
+-  Address details
+-  Photo
+-  Relation to guardians
+-  Primary group (e. g. a class or a tutor group, cf. [@sec:Groups])
+
+Except for the name, all data points are optional, so you can decide on
+your own (and on your local data protection laws) which data should be
+included in AlekSIS.
+
+There are two important things you should know about persons:
+
+-  **Persons are not automatically users:** That means that persons can
+   be linked to an user account including things like a password and the
+   ability to login, but they don’t have to be. For example, your
+   AlekSIS instance could save the data about parents, but you don’t
+   want them to login: In this scenario, the guardians are available as
+   persons **without** user accounts.
+-  **Persons are not linked to school terms:** As persons like students
+   are not only at the school for one school term, persons are not
+   linked to school terms.
+
+Manage persons
+~~~~~~~~~~~~~~
+
+The main method to manage persons is the view under
+``People → Persons``. To add person to groups, you have to open the
+respective group and set the person as a member or an owner.
+
+.. _sec:Groups:
+
+The concept of groups
+---------------------
+
+The AlekSIS groups are a universal way to organise persons in
+collections like classes, courses, tutor groups, clubs, or any other
+division you could imagine. They track the following data:
+
+-  Group name and short name
+-  Owners (e. g. class or course teacher(s))
+-  Members (e. g. students)
+-  Parent groups (e. g. a class could be a parent group for a course)
+-  Group type (e. g. class, course, club, etc.)
+
+In contrast to persons, groups are supposed to be **linked to school
+terms** (but they don’t have to be). For example, the composition of a
+class or a course varies from school term to school term. In order to
+archive historical data according to local laws, these groups have to be
+separeted which is solved by linking them to a school term.
+
+Manage group types
+~~~~~~~~~~~~~~~~~~
+
+You can manage your local group types by opening the menu entry
+``People → Group types`` as an admin user.
+
+Manage groups
+~~~~~~~~~~~~~
+
+Groups are managed on the page ``People → Groups``. There you can
+search, view, create, change and delete groups.
+
+Import school terms, persons and groups from other data sources
+---------------------------------------------------------------
+
+When AlekSIS is not your single date source, all these data can be
+imported from other sources. You can find further information in
+[@sec:Integrations].