Custom email domain guide: remove SPF mentions

[brentschaus-okta]

Description:

• What's changed? Removes some mentions of SPF in the custom email domain guide. Adds link to use your own email provider doc.
• Is this PR related to a Monolith release? No.

Resolves:

• OKTA-869591

[acrolinxdevdocs-okta]

Acrolinx score

A minimum Acrolinx Score of 80 is required. The total score is an average of the subscores.

Select Total score to review the Acrolinx scorecard for your article. Try to increase your individual scores, for example: Correctness. Your content will be clearer and more consistent.

Article	Total score Required:80	Word and phrases (Brand, terms) Preferred: 80	Correctness (Spelling, grammar) Preferred: 80	Clarity (Readability) Preferred: 80	Inclusive language (+ accesibility) Preferred: 80
✅ packages/@okta/vuepress-site/docs/guides/custom-url-domain/main/index.md	80	48	80	87	99

• Scorecard links, by default, are only good for 7 days. Link for this commit's Content Analysis Report

[eng-info-dev-github-bot]

Netlify Preview URL for the changes: https://preview-5335--reverent-murdock-829d24.netlify.app

[acrolinxdevdocs-okta]

Acrolinx score

A minimum Acrolinx Score of 80 is required. The total score is an average of the subscores.

Select Total score to review the Acrolinx scorecard for your article. Try to increase your individual scores, for example: Correctness. Your content will be clearer and more consistent.

Article	Total score Required:80	Word and phrases (Brand, terms) Preferred: 80	Correctness (Spelling, grammar) Preferred: 80	Clarity (Readability) Preferred: 80	Inclusive language (+ accesibility) Preferred: 80
✅ packages/@okta/vuepress-site/docs/guides/custom-url-domain/main/index.md	80	48	80	87	99

• Scorecard links, by default, are only good for 7 days. Link for this commit's Content Analysis Report

[acrolinxdevdocs-okta]

Acrolinx score

A minimum Acrolinx Score of 80 is required. The total score is an average of the subscores.

Select Total score to review the Acrolinx scorecard for your article. Try to increase your individual scores, for example: Correctness. Your content will be clearer and more consistent.

Article	Total score Required:80	Word and phrases (Brand, terms) Preferred: 80	Correctness (Spelling, grammar) Preferred: 80	Clarity (Readability) Preferred: 80	Inclusive language (+ accesibility) Preferred: 80
✅ packages/@okta/vuepress-site/docs/guides/custom-url-domain/main/index.md	80	48	80	87	99

• Scorecard links, by default, are only good for 7 days. Link for this commit's Content Analysis Report

[acrolinxdevdocs-okta]

Acrolinx score

A minimum Acrolinx Score of 80 is required. The total score is an average of the subscores.

Select Total score to review the Acrolinx scorecard for your article. Try to increase your individual scores, for example: Correctness. Your content will be clearer and more consistent.

Article	Total score Required:80	Word and phrases (Brand, terms) Preferred: 80	Correctness (Spelling, grammar) Preferred: 80	Clarity (Readability) Preferred: 80	Inclusive language (+ accesibility) Preferred: 80
✅ packages/@okta/vuepress-site/docs/guides/custom-url-domain/main/index.md	80	48	80	87	99

• Scorecard links, by default, are only good for 7 days. Link for this commit's Content Analysis Report