docs: add introduction to Blockchain Indexing

[Davphla]

Close: #2295

Minimal Documentation to introduce Blockchain indexing through tx-indexer. I've as well made made an introduction video with a GraphQL live coding example : https://www.youtube.com/watch?v=8ld0nsV37SA.

[Gno2D2]

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[n0izn0iz]

I think you should use something else than balance as an example, since for that you can simply query the chain.

I think a good example is building a search tool.

Example, for social content: indexing blog posts and building a full-text-search feature over posts content on top of that, which is something that would cost a huge amount of gas if done on-chain. Also search for usernames is a good example.

[Davphla]

@n0izn0iz I modified the section to add more detailed and relevant example following your feedback! :+1:

[leohhhn]

Hey @Davphla, let me know when you are ready for another round of reviews.

[Davphla]

I re-open it. I reworked it since it was not really great before.

I add a full example implementation step by step (i tried it it's all working great, I can upload it in a separate repository). It's a lot of code in the documentation, but I believe in that specific case that's the most efficient way to share knowledge. It's quite hard to implement when you don't know anything about it (as I got when writing this documentation). I'm open to listen any changes and review!

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:white_check_mark: All modified and coverable lines are covered by tests.

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[Davphla]

Code available at: https://github.com/samouraiworld/tx-indexer-example (To be moved in Gnoverse).

I've modified through the new update:

• Remove the database part from the documentation. I think is unrelated to the use of the indexer -- Though it is still available through the link above.
• Use of embedmd to extract the code to the _assets folder.
• Use a json structure to match the GraphQL structure