chore: untrack resources/_gen/ (regenerable Hugo asset cache)

These compiled-SCSS cache files were being versioned but are regenerated
on every build (each build produces new hash-named files and orphans the
old ones), so they churned the working tree on every commit. They are now
gitignored (see the .gitignore change in the build-tooling commit) and
untracked here. Nothing references them at runtime; Hugo rebuilds them
from book.scss as needed.

resources/_gen/assets/book.scss_b807c86e8030af4cdc30edccea379f5f.json

@@ -1 +0,0 @@
-{"Target":"book.min.0edbfa12be0756ec13c02ec8218ae882d32e82be873db98b790ade0a47d58b55.css","MediaType":"text/css","Data":{"Integrity":"sha256-Dtv6Er4HVuwTwC7IIYrogtMugr6HPbmLeQreCkfVi1U="}}

resources/_gen/assets/book.scss_e129fe35b8d0a70789c8a08429469073.json

@@ -1 +0,0 @@
-{"Target":"book.min.bd0c2ca7caf6a870ed62ce6a734b0bcdf2e612df263b13b2c9c55e48ab315ad3.css","MediaType":"text/css","Data":{"Integrity":"sha256-vQwsp8r2qHDtYs5qc0sLzfLmEt8mOxOyycVeSKsxWtM="}}

resources/_gen/assets/scss/book.scss_50fc8c04e12a2f59027287995557ceff.json

@@ -1 +0,0 @@
-{"Target":"book.min.4b35fed0bea034bbc19c89c71e14b73fb9c68cfcc586b9382adfb9b7b103ba06.css","MediaType":"text/css","Data":{"Integrity":"sha256-SzX+0L6gNLvBnInHHhS3P7nGjPzFhrk4Kt+5t7EDugY="}}