Clean up and consolidate .gitignores

* Global ignores at toplevel .gitignore
* Component-specific ignores in each toplevel directory
* Remove `actionview/test/tmp/.keep` for JRuby

```
rm actionview/test/tmp/ -fr
cd actionview/
bundle exec jruby -Itest test/template/digestor_test.rb
```

Related to #11743, #30392.

Closes #29978.

.gitignore

-# Don't put *.swp, *.bak, etc here; those belong in a global ~/.gitignore.
+# Don't put *.swp, *.bak, etc here; those belong in a global .gitignore.
 # Check out https://help.github.com/articles/ignoring-files for how to set that up.
 
 .Gemfile
-.ruby-version
 .byebug_history
-debug.log
-pkg
+.ruby-version
+/*/doc/
+/*/test/tmp/
 /.bundle
-/dist
-/doc/rdoc
-/*/doc
-/*/test/tmp
-/activerecord/sqlnet.log
-/activemodel/test/fixtures/fixture_database.sqlite3
-/activesupport/test/fixtures/isolation_test
-/activestorage/test/service/configurations.yml
-/railties/test/500.html
-/railties/test/fixtures/tmp
-/railties/test/initializer/root/log
-/railties/doc
-/railties/tmp
-/guides/output
+/dist/
+/doc/
+/guides/output/
+debug.log
 node_modules/
-/actionview/log
+package-lock.json
+pkg/

actioncable/.gitignore

-/lib/assets/compiled
-/tmp
+/lib/assets/compiled/
+/tmp/

actionview/.gitignore

-/lib/assets/compiled
-/tmp
+/lib/assets/compiled/
+/log/
+/test/fixtures/public/absolute/
+/test/ujs/log/
+/tmp/

actionview/test/fixtures/public/.gitignore

-absolute/*

actionview/test/ujs/.gitignore

-/log

activerecord/.gitignore

+/sqlnet.log
+/test/config.yml
+/test/fixtures/*.sqlite*

activerecord/examples/.gitignore

-performance.sql

activerecord/test/.gitignore

-/config.yml

activerecord/test/fixtures/.gitignore

-*.sqlite*
\ No newline at end of file

activestorage/.gitignore

-.byebug_history
-node_modules
-src
-test/dummy/db/*.sqlite3
-test/dummy/db/*.sqlite3-journal
-test/dummy/log/*.log
-test/dummy/tmp/
+/src/
+/test/dummy/db/*.sqlite3
+/test/dummy/db/*.sqlite3-journal
+/test/dummy/log/*.log
+/test/dummy/tmp/
+/test/service/configurations.yml

activesupport/.gitignore

+/test/fixtures/isolation_test/

railties/.gitignore

-log/
+/log/
+/test/500.html
+/test/fixtures/tmp/
+/test/initializer/root/log/
+/tmp/