Add migration guides

[Siegrift]

Closes api3dao/airnode#1448

• For the 0.8 migration guide, I copied over the migration guide from @andreogle. See the original slack thread in https://api3workspace.slack.com/archives/C02AYRX8D89/p1663596496939549
• For the 0.9 migration guide, I copied over the content from Migration from 0.8 to 0.9 Airnode airnode#1461 and also resolved comments from @andreogle

[dcroote]

Nice 👍 only very minor rephrasing suggestions from me.

[Siegrift]

In 0.9. we also have Sepolia network available for Airnodes to use. I don't think this needs to be mentioned in the migration guide though.

[wkande]

I recall @dcroote mentioned some issues with the 0.7.0 to 0.8.0 guide by Andre. I will take a look into them after merging which is when I will add your guides to the sidebars.

• #4 - might be worth mentioning rrp[n].cacheResponses is a new, required field
• Refer to airnode-admin wherever applicable #5 (both in the summary and the Details): 0.8.0 uses oisFormat "1.1.1" rather than "1.1.2" (see here for example). There is a bit of debate on how to handle the coupling of Airnode and OIS going forward though (for which we might need a 4th opinion 😅).
• Minor typo "0.8.9" at the end
• corsOrigins was added for HTTP gateways (relevant config.json changes here) in #1357.
• relayRequestId was added as a supported security scheme. Functionality implemented with an example of use in #1286 (though between that PR and now, OIS was moved into its own repo).

[dcroote]

@wkande - Andre incorporated all of my changes before @Siegrift copied it for this PR so all set.

@Siegrift - I think we should mention Sepolia is available and that we're deprecating some testnets (api3dao/airnode#1476), including why we're doing it. This way we can point users (e.g. from Discord) to one piece of information about the change.

[dcroote]

Requesting updates per my above comment

[Siegrift]

I think we should mention Sepolia is available and that we're deprecating some testnets (api3dao/airnode#1476), including why we're doing it. This way we can point users (e.g. from Discord) to one piece of information about the change.

Ok. Added in last commit. Please either @wkande or @dcroote take a look at wording and make any necessary updates. Feel free to merge it afterwards.

[github-actions]

Visit the preview URL for this PR (updated for commit eb302d3):

https://api3-docs--pr1056-migration-guides-gnvlc8mt.web.app

_{(expires Thu, 06 Oct 2022 05:01:29 GMT)}

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[dcroote]

Thanks @Siegrift. I added a note about Etherscan discontinuing their blockchain explorers and also added these guides to the sidebar nav.

@wkande I'll leave it to you to merge