Add bookmarking information to vignettes

Companion to https://github.com/insightsengineering/teal/pull/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](https://github.com/insightsengineering/coredev-tasks/issues/520) and [this PR](https://github.com/insightsengineering/teal/pull/1011) 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
```