feat: add alt for images and add some semantic tags

[henrikvilhelmberglund]

Description

Please describe what changes you made, and why you feel they are necessary. Make sure to include
code examples, where applicable.

This PR:

• adds alt for sponsor images which had missing alt attributes
• changes header div to a header element
• changes nav div to a nav element
• changes main div to a main element

Feel free to make changes.

Corresponding issue:
N/A

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded

[1cg]

gottem