Add more descriptive pages about the Portal's operations

[jrdh]

The most sensible place for this is likely under https://data.nhm.ac.uk/about, perhaps in a new section?
Then it can sit next to the newly updated citation page.
I don't think this should approach user manual type pages at this point but it would be excellent to be able to point at some written documentation about core functionality on the Data Portal.

Topics we could provide some details about:

• versioning - where does it apply, what does it mean, how does it work (less important)
• permanent URLs - /object/ for specimens, /record/ for all other records, /media/ for all collection images

What other topics would be worth covering?

Could we use markdown to make these easier to write and update?