doc/start: explain "OSD"

doc/start: explain "OSD"

Explain the initialism "OSD" and link to its definition in the glossary.
This PR is raised in response to an anonymous documentation bug that reads

  "Paragraph 2 uses the acronym OSD without any explanation.
   This makes it very difficult to understand this part of
   the documentation as there is no indication of what this
   acronym is until much further into the documentation. Replace
   first occurence of OSD with Object Storage Daemon (OSD) or
   link it to the glossary."
     -- https://pad.ceph.com/p/Report_Documentation_Bugs

Co-authored-by: Anthony D'Atri <anthony.datri@gmail.com>
Signed-off-by: Zac Dover <zac.dover@proton.me>