Fix alignments issues on pages on the mobile design

[HaudinFlorence]

There are some centering issues on most of the components of the various page:

• home
• projects (not on the header but on the components introducing the various projects)
• about us page
• careers: section with the video images
• blog: the page is ok when dezooming a little but not centered otherwise.
• contact us: left margin seems a bit wider

[HaudinFlorence]

This has been made better with https://github.com/QuantStack/quantstack.github.io/pull/139.