Port CASTOR tape format/tape server documentation to EOSCTA docs

The CASTOR tape server is documented here:

https://gitlab.cern.ch/cta/CTA-old/-/blob/master/doc/TapeServer.pdf

Some parts of this document also apply to CTA. In particular, the CASTOR tape file format is defined on p.18-21 and this is identical to the format used for CTA.

The relevant parts of the documentation should be identified and copy/pasted to eoscta.docs.cern.ch.