Managing kong.conf reference & how-to

[lena-larionova]

Description

This was an interesting one: I started it thinking it would just be a reference, but realized just how many times in the past week I've seen us all stumble at applying kong.conf to an instance.

Added the how-to as part of this PR instead of splitting it out because it's a distilled/simpler version of some of the info in the reference. Reviewers, let me know what you think of this approach and if you think both pages are helpful.

Fixes #287

Preview Links

https://deploy-preview-375--kongdeveloper.netlify.app/how-to/configure-and-apply-kong-conf/
https://deploy-preview-375--kongdeveloper.netlify.app/gateway/manage-kong-conf/

Checklist

• Every page is page one.
• Tested how-to docs. If not, note why here.
• All pages contain metadata.
• Updated sources.yaml. For more info, review track docs changes
• Any new docs link to existing docs.
• All autogenerated instructions render correctly (API, decK, Konnect, Kong Manager).
• Style guide (capitalized gateway entities, placeholder URLs) implemented correctly.

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