Update examples.md for 1.1 release

[arouinfar]

For #150

The examples doc is in need of extensive rewriting. I'll use this issue to reference the commits and explain the changes.

[arouinfar]

In the above commit, I removed 3 examples, all containing deprecated information.

1. Hide each item in the navigation bar
2. Select which screen to start on
3. Change the labels on the blocks

Examples (1) and (2) highlight customizations that we don't necessarily recommend, but were previously requested by a client. Much of the content is outdated, and anything still relevant is covered in the PhET-iO Guide.

Example (3) was removed because the process for changing strings has drastically changed.

[arouinfar]

I updated the Density examples doc in the above commits.

@DianaTavares can review it? Please try the customizations I describe in the doc and let me know if you run into any issues or have suggestions.

[arouinfar]

@DianaTavares's review is in #158 and no changes were necessary.