Placeholder for new OpenSAFELY Data page

[benbc]

Will is writing a new page for the OpenSAFELY documentation that describes the data that will be available to researchers for non-COVID research. (There is a draft here [Bennett-internal only].)

This page will be the canonical location to link to from other documentation whenever we need to describe the available data and any restrictions on accessing it (for example from the ehrQL schema or the CLI docs).

It's going to be some time before this page is finished because the topic is complex and needs discussion with and approval from a lot of different people. But we would like to get started on writing various bits of documentation sooner rather than later.

To resolve this dependency, we've agreed to create an empty placeholder page, so there is known URL to link to from the other documentation that can be populated once the text has been agreed.

This piece of work is complete when:

• there is a page called "OpenSAFELY Data" in the docs
• the new page appears in the nav menu in the "About OpenSAFELY" section
• the text of the page reads "Detailed description of data available in OpenSAFELY coming soon."
• Lizzie has been given the URL of the new page for linking from the new website
• Will has been given the URL of the new page for populating with the text once it's ready

Linking to this new page from the appropriate places elsewhere in the documentation is out of scope for this issue, as is getting the actual text published.

For the avoidance of doubt: the agreed plan is for this blank page to be actually published to the live docs site, not held back in a draft PR.