Update contribute page, closes #74 and #75

[Twissi]

• [x] copy of this section needs to be slightly change.
• [x] remove "foundation"
• [x] modify copy so is clear we are always looking for volunteers
• [x] highlight that people support us with time
• [x] spirit about “come and do” more prominent
• [x] in "Sponsor" text, talk how sponsor can contact us. (info@opentechschool.org)
• [x] in "Sponsor" comment out everything related to donation.

Also updates design of volunteer section and adds illustration (from https://github.com/OpenTechSchool/websitenext/issues/75)

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[Twissi]

Not sure if we maybe want to remove the membership from the section. Let me know what you think @gsambrotta

[Twissi]

@gsambrotta updated!