docs: clarify 'How does TUF secure updates?' for first-time readers - #165

Changed pages include:

• index.html - view
• 404.html - view
• about/index.html - view
• community/index.html - view
• docs/index.html - view
• resources/index.html - view
• search/index.html - view
• spec/index.html - view
• community/adoptions/index.html - view
• community/taps/index.html - view

There are more changes not included in the list