fix(docs): Removed custom-indexer URLs and added admonition

[lucas-tortora]

Description of change

Remove the URLs in the custom indexer dev docs as they are not available at the moment.

Type of change

• Documentation Fix

How the change has been tested

Docs were built locally.

Change checklist

Tick the boxes that are relevant to your changes, and delete any items that are not.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have checked that new and existing unit tests pass locally with my changes

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[vivekjain23]

LGTM