Update Blob Documentation to include example for getBlobContent

[biozal]

Per the email I sent out, the blob documentation is missing the code example and explanation on how to get the blob's content from a given document.

Review the code that decodes a blob attachment that is text here:
https://github.com/Couchbase-Ecosystem/cbl-ionic/blob/main/example/src/pages/documents/GetDocument.tsx#L34

The blob for text is encoded here:
https://github.com/Couchbase-Ecosystem/cbl-ionic/blob/main/example/src/pages/documents/EditDocument.tsx#L74

Also the original documentation shows an example of doing this via images - although it uses some hack window root scope example that we shouldn't be including in our code, but the text example below the code is good:
https://ionic.io/docs/couchbase-lite/usage#blobs