Migrate GLM section from FSL wiki

Migrate the GLM section from the FSL wiki: https://fsl.fmrib.ox.ac.uk/fsl/fslwiki/GLM

The suggested group for this section is Statistics:

• The section content should be migrated into docs/statistics/glm.md.
• An entry for the new page should be added to docs/statistics/_sidebar.md.

This section has the following subpages on the old wiki:

• GLM/CreatingDesignMatricesByHand
• GLM
• GLM/Faq
• GLM/Contents

Attachments for this section can be found at:

• https://git.fmrib.ox.ac.uk/paulmc/fsl-docathon-2023/-/blob/main/fsl-wiki-dump-20230817/attachments/GLM