OHW24 volunteer recruitment page and related OHW24 updates

[emiliom]

• Created new ohw24 site (/ohw24 directory) with a single page (the index page) based on the content of the Google Doc we drafted for recruiting volunteers for ohw24. On that page:
  • Added link to Hubspot form
  • Added some section organization
  • Where ohw "mission & vision" and code of conduct were mentioned, I turned them into links.
• Updated the front page call out to include the ohw24 dates and point to the volunteer recruitment page
• On the top bar, replaced the link to ohw23 with ohw24

Note: I've removed the outdated About > Task Groups page. See #311

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