Add partnerships page and move partnerships section

[kjaymiller]

Summary

• Created a dedicated partnerships page at /partnerships.html
• Converted partnerships data from YAML to JSON format (_data/partnerships.json)
• Added Rodrigo Girão Serrão as a new partner with promo code BLACKPYTHONDEVS
• Removed partnerships section from homepage and moved to dedicated page
• Added "Partnerships" link to footer navigation and main nav
• Added partnership logo image for Rodrigo

Test plan

• Verify partnerships page renders correctly at /partnerships.html
• Check that all three partnerships display properly with logos and descriptions
• Confirm "Partnerships" navigation link appears in footer
• Verify homepage no longer shows partnerships section
• Test all partnership links and promo codes work correctly

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