Update projects.html with free tools for security.txt generator and validator

[apalyekar]

Adds the security.txt Index (RFC 9116 adoption and validity tracker for the top 500 sites) plus the free generator and validator tools. Placed at the top of the table to match the newest first ordering.

[apalyekar]

This adds three security.txt (RFC 9116) resources to the projects list:

• security.txt Index: a browsable, sortable index tracking security.txt adoption across the top 500 sites by traffic. It validates each file against RFC 9116, flags expired files and missing Expires fields, notes PGP signing, and offers the full dataset as a CSV. https://www.sitesecurityscore.com/web-security-index

• security.txt Generator: a free tool that produces a valid RFC 9116 file.
  https://www.sitesecurityscore.com/tools/security-txt-generator

• security.txt Validator: a free tool that checks an existing file against RFC 9116.
  https://www.sitesecurityscore.com/tools/security-txt-validator

Happy to adjust the wording or trim down to just the index if you prefer.