Migrate topup section from FSL wiki

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

The suggested group for this section is Diffusion MRI:

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

This section has the following subpages on the old wiki:

• topup/ApplytopupFurtherInformation
• topup/Contents
• topup/TopupUsersGuide
• topup/UserGuide
• topup/ApplyTopupUsersGuide
• topup
• topup/ExampleTopupFollowedByApplytopup
• topup/TopupFurtherInformation
• topup/Faq

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/topup(2f)TopupUsersGuide
• https://git.fmrib.ox.ac.uk/paulmc/fsl-docathon-2023/-/blob/main/fsl-wiki-dump-20230817/attachments/topup(2f)Faq
• https://git.fmrib.ox.ac.uk/paulmc/fsl-docathon-2023/-/blob/main/fsl-wiki-dump-20230817/attachments/topup(2f)ApplytopupFurtherInformation