Create a topic about fixtures

[Filip Łajszczak]

Background: From working on #26975 and from reading discussion on the ​first PR it looks like the current documentation of fixtures requires some work.

Currently information about fixtures is dispersed between different parts of the docs:

• general definition of fixtures lives under reference for loaddata ​command
• example of the hand written fixture is in the ​Initial data how-to guide
• other bits one can find in ​testing tools

There are places where fixtures are mentioned without any prior explanation and without any link to more information, like in dumpdata ​command docs

There are places where fixtures should be mentioned by name but are not, like ​seriaIization Topic

Looks like creation of the separate Topic under ​Models and databases could be the solution.