Rename Parathreads -> On-demand Parachains

[filippoweb3]

• Rm parathread page (update sidebar and add redirects) and move content to learn-parachains
• Swap parathread -> on-demand parachains
• Make sure there is a connection between on-demand parachains and Agile Coretime

[filippoweb3]

@CrackTheCode016 let me know if these changes make sense to you. See the attached issue by @Overkillus

[CrackTheCode016]

Very good refactor, thank you! We can further improve more of the explainations in further PRs, but this is really great to clarify right now 🙏

The only reason I did not approve was the use of the word "permanent" - I'm not sure it is entirely accurate, perhaps it would be better to allude to bulk coretime instead. However, I could be wrong here.

[CrackTheCode016]

Curious if this is indeed the case, not sure how queue is managed in relation to on-demand (I suspect it's the same, still good to check probably though)

[filippoweb3]

not sure, I did not touch that sentence if not just for swapping parathread <-> on-demand parachain

[filippoweb3]

@CrackTheCode016 I implemented your feedback. Added @DrW3RK as a secondary reviewer to provide final feedback before merging.

[CrackTheCode016]

Looks overall fine, thanks! I see that "permanent" is still being used in a few places, however I will leave the final review to Radha to decide.

[filippoweb3]

@DrW3RK thanks for the final review. I noticed you added coretime in a few pages although not deployed on Polkadot yet. I approved all suggestions and there will be a few places still mentioning auctions. Resolved a few conflicts and merging this.

[DrW3RK]

Right. Considering these docs also render on the Kusama guide, I mentioned coretime where necessary (especially, with on-demand parachains which only make sense with coretime in picture). We can change those to conditionally render through a follow-up PR (if it is worth the effort) or wait till Agile Coretime goes live on Polkadot to just eliminate all the references to slots/leases when.