Featuring Indexers in Tutorials

[bgodlin]

Hello Cosmos team, We are submitting this contribution with the intention of adding a new chapter to the tutorials section, focusing on indexers, which play a fundamental role in application development. It also includes the SubQuery indexer as one of the user options.

Kindly inform us if any adjustments are necessary on our part.

Change scope

The changes in this Pull Request include (please tick all that apply):

• [ ] Small language/grammar fixes
• [ ] Small content fixes
• [x] Addition of new content
• [ ] Sample code/command updates
• [ ] Platform fixes
• [ ] Other