Add alt text to images and fix minor typos - #523

Changed pages include:

• 404.html - view
• docs/index.html - view
• docs/coffee-table-groups/index.html - view
• docs/coffees/index.html - view
• docs/lightning-talks/index.html - view
• docs/lunch-and-learns/index.html - view
• docs/monthly-challenges/index.html - view
• docs/vc-conference/index.html - view
• docs/vchi/checklist/index.html - view
• docs/vchi/messages/index.html - view

There are more changes not included in the list