Update docs in ibm-db-haystack integration to use IBM Db2 instead of IBM Db

[sjrl]

Based on this comment deepset-ai/haystack#11925 (review)

we should

1. update the integration docs such that the auto-generated front matter in our API docs go from

title: "Ibm Db"
id: integrations-ibm-db
description: "Ibm Db integration for Haystack"

to

title: "Ibm Db2"
id: integrations-ibm-db
description: "Ibm Db2 integration for Haystack"

and

2. Update all occurrences of DB2 to Db2 to reflect the correct name

[avsingh23]

@sjrl what is the next step from our side?

[sjrl]

Hey @avsingh23 it should be fixed now in #3574 and deepset-ai/haystack#11946