Update Gitbook documentation for fetching records from Sierra

[pollecuttn]

'Fetching records from source catalogues' has documentation for CALM and MIRO but not Sierra.

Alex pointed out that some of that exists at https://github.com/wellcomecollection/catalogue-pipeline/tree/main/sierra_adapter and that the info needs to be updated.

We also need information in Gitbook about fetching records from Sierra staging. See https://wellcome.slack.com/archives/C05JE6PE5PG/p1693316612890959 for more info on that.

NB API keys for Sierra prod and staging are managed by Collections System Support in D&T.

TODO

• [ ] Add info to Gitbook to include fetching records from Sierra, and make sure it's up-to-date
• [ ] Add in information for using Sierra staging