Add async backing technical reference

[charlesHetterich]

• still need to finalize diagrams and give better context of them

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[Copilot]

Pull Request Overview

This PR adds new documentation for Polkadot's asynchronous backing and inclusion pipeline features, explaining how rollup blocks are processed and validated in the relay chain.

• Introduces comprehensive documentation for asynchronous backing, including configuration options and visual comparisons between synchronous and asynchronous processing
• Adds a new page explaining the inclusion pipeline stages (Generation, Backing, and Inclusion)
• Updates the llms.txt index to reflect the new documentation pages

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 9 comments.

File	Description
reference/parachains/consensus/inclusion-pipeline.md	New documentation page explaining the multi-step pipeline for rollup block processing, including Generation, Backing, and Inclusion stages
reference/parachains/consensus/async-backing.md	New comprehensive documentation on asynchronous backing, including governance configurations, processing diagrams, and multi-core scenarios
llms.txt	Updated documentation index with new page entries and incremented page count from 269 to 271

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[charlesHetterich]

This is ready for feedback. I split this up into two technical reference pages

• Async Backing
• Inclusion Pipeline

Everything is in good condition besides the section titled Synchronous VS. Asynchronous Processing. Before I continue spending time on it, I wanted to get feedback how to structure this section, where it should live, or if it should just be removed entirely. The intention of the section is to provide a visual understanding of what async backing is.

Additionally there are 3 empty links in inclusion-pipeline.md

• candidate receipt
• erasure code chunks
• approval process (in reference to the final approval process of the inclusion pipeline)

Which, I was hoping there are definition these could link to?

[brunopgalvao]

Left my review. Overall I would focus on making sure the concept of async backing flows well for readers. I left some comments that can help in the direction.

[nhussein11]

LGTM! Left some minor comments but approved :) @charlesHetterich, please ensure you mark the conversions as solved once you've fixed those comments, whether Bruno's or mine.