Add bookmarking information to vignettes

[chlebowa]

Companion to #1011

1. Add new vignette that explains bookmarking.

• explain that teal bookmarks teal and modules bookmark themselves
• explain the teal_bookmarkable attribute of modules
• use this issue and this PR as sources of content

2. Amend Creating Custom Modules vignette.

• mention bookmarking requirements and attribute
• link to bookmarking vignette

3. Amend pkgdown.
   suggested:

- title: Extending `teal`
  navbar: Extending `teal`
  contents:
    - creating-custom-modules
    - bookmarking-teal-applications
    - adding-support-for-reporting

[donyunardi]

Since we're not exposing the feature just yet, I think we can postpone the vignette until later.
Or is there any specific reason to introduce the vignette now?

[chlebowa]

I should be the one to write it.

[donyunardi]

Agree, we can write this for now and hide it from public until we're ready to expose it.