What

This MR adds documentation for the contents of the _tobi directory, which contains Tobi's cache.

Why

This knowledge can be useful to debug issues with installed components.

Wrote the page and added it to the index of Tobi's documentation.

You can read the markdown version in GitLab's interface. In the diff view, click on the three dots and select "View file @ (commit hash)":

To read the HTML version:

sudo apt-get install markdown
cd tobi/doc
make
open html/index.html

Document the interface of any function added or modified (see the coding guidelines)
Document any change to the user interface, including configuration parameters (see node configuration)
Provide automatic testing (see the testing guide).
For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
Select suitable reviewers using the Reviewers field below.
Select as Assignee the next person who should take action on that MR