add help centre domain cnames (temporary values for authentication)

[johnduffell]

We would like to serve the salesforce experience version of the help centre via fastly.

The CODE version is here: https://help.code.dev-theguardian.com/

This requires quite a bit of setup, first we have to prove to salesforce that we own the domain by pointing help.theguardian.com to a special value, then we have to change it to correctly point it to fastly.

Relevant docs:
https://help.salesforce.com/s/articleView?id=platform.domain_mgmt_enable_https.htm&type=5
https://help.salesforce.com/s/articleView?id=platform.custom_url_add.htm&type=5

CODE only:

• Since the CODE CNAME record was created manually in NS1, we first need to "adopt" the record to CFN by creating it with the same details (done)

CODE and PROD

• We need to CNAME the host name to a special (invalid) hostname, presumably to prove to salesforce that any incoming traffic with the host header of our customer facing domain is actually supposed to be picked up by our salesforce backend (and we are not stealing traffic from some unrelaited salesforce customer who is on the same tenant)
• Then we need to update it with the correct fastly name ( point help centre at fastly #1713 )

This PR adds the Guardian DNS record entry needed to authenticate, (then we can do another PR to change it to the correct fastly hostname.)

Testing:

Deployed to CODE and the record is available:

% dig +noall +answer CNAME help.code.dev-theguardian.com
help.code.dev-theguardian.com. 300 IN    CNAME    help.code.dev-theguardian.com.00d20000000nq5geaa.live.siteforce.com.
%

and the domain could be added in salesforce successfully.

Then the next PR #1713 was also deployed to CODE, at which point the site started working

[github-actions]

Deploy build 1946 of support::manage-frontend to CODE

All deployment options

• Deploy build 1946 of support::manage-frontend to CODE
• Deploy parts of build 1946 to CODE by previewing it first
• What's on CODE right now?

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From guardian/actions-riff-raff.

[github-actions]

Deploy build 1946 of support::help-centre to CODE

All deployment options

• Deploy build 1946 of support::help-centre to CODE
• Deploy parts of build 1946 to CODE by previewing it first
• What's on CODE right now?

---

From guardian/actions-riff-raff.

[rafaelbarreto-theguardian]

LGTM

[gu-prout]

Seen on PROD (merged by @johnduffell 8 minutes and 37 seconds ago) Please check your changes!