OIDC: On-prem guide for keycloak and client creds grant

[lena-larionova]

Description

First how-to guide for configuring the OIDC plugin with the client credentials grant.

Partially fixes #390, this is only one config. There's many more.

Notes/questions:

• This config doesn't work in Konnect. I still need to figure out that out; I just learned after multiple days of testing that Konnect doesn't support the OIDC discovery endpoint, so the client auth method used in all the examples doesn't work in Konnect at all.

Preview Links

Checklist

• Every page is page one.
• Tested how-to docs. If not, note why here. - still figuring out Konnect
• 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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[lena-larionova]

Setting to draft as now I can't get this to work, and it isn't working in Konnect. Going to take another approach and look for ways to write this so that it works on both deployment types.