Contribution description

This is yet another PR for improving the documentation structure.

Here I group several math modulesin a Math libraries and utilities group, so they are easier to find for the end user.

A sys_math group was defined under sys group.

The following modules where touched and added to the corresponding groups:

• sys/div.h: sys_math
• sys/matstat: sys_math
• sys/seq: sys_math
• pkg/libfixmath: sys_math, pkg

Testing procedure

make doc.

Go to System>Math libraries and utilities in the generated documentation and see how these components are grouped.

Issues/PRs references

#8479