build(docs-infra): renamed e2e property of example-config.json to tests (#36143)

Each docs example has an `example-config.json` configuration file. Among
other things, this file can be used to specify what commands to run in
order to test the example. (If not specified, the `run-example-e2e.js`
script will run a default `yarn e2e` command.)

Previously, the property specifying the test commands was called `e2e`.
This is because in the past only e2e tests were run for docs examples.
Since recently, some examples may specify commands for other types of
tests (such as unit tests). Therefore, calling the property that holds
the list of test commands `e2e` no longer makes sense and can be
misleading to people looking at the configuration files.

This commit renamed the property to the more generic `tests`. In the
future, the `run-example-e2e.js` script (and corresponding npm script)
should be renamed and refactored to also avoid giving the impression
that only e2e tests are run.

Discussed in:
https://github.com/angular/angular/pull/36143#discussion_r395148379

PR Close #36143

aio/content/examples/rx-library/example-config.json

@@ -1,5 +1,5 @@
 {
-  "e2e": [
+  "tests": [
     {
       "cmd": "yarn",
       "args": [ "tsc", "--project", "./tsconfig.app.json" ]