chore: Check that all blog articles are signed in CI (#1459)

* Check that all blog articles are signed in CI

Signed-off-by: Alexander Rodin <rodin.alexander@gmail.com>

* Add summary comment to `check-blog-signatures.rb`

Signed-off-by: Alexander Rodin <rodin.alexander@gmail.com>

* Add a clarifying comment

Signed-off-by: Alexander Rodin <rodin.alexander@gmail.com>

* Use local keyring directories

Signed-off-by: Alexander Rodin <rodin.alexander@gmail.com>

* Add blog signature

Signed-off-by: Ashley Jeffs <ash@jeffail.uk>

* Don't check *.sig files for newlines

Signed-off-by: Alexander Rodin <rodin.alexander@gmail.com>

* Sign Ben\'s blog posts

Signed-off-by: binarylogic <bjohnson@binarylogic.com>

* Add new `make sign` target

Signed-off-by: Alexander Rodin <rodin.alexander@gmail.com>

* sign intro to vector post

Signed-off-by: Luke Steensen <luke.steensen@gmail.com>

* Update front matter for new blog articles

Signed-off-by: Alexander Rodin <rodin.alexander@gmail.com>

* chore: Add new blog sigs

Signed-off-by: Ashley Jeffs <ash@jeffail.uk>

Co-authored-by: Ashley Jeffs <ash@jeffail.uk>
Co-authored-by: Binary Logic <bjohnson@binarylogic.com>
Co-authored-by: Luke Steensen <luke.steensen@gmail.com>