RHIDP-6970: Fix headings in Customizing doc

[linfraze]

IMPORTANT: Do Not Merge - To be merged by Docs Team Only

Version(s):
1.5
Issue:
https://issues.redhat.com/browse/RHIDP-6970
Preview:
https://redhat-developer.github.io/red-hat-developers-documentation-rhdh/pr-1071/customizing/#proc-customize-rhdh-sidebar-menuitems_customizing-appearance

[rhdh-bot]

Preview: https://redhat-developer.github.io/red-hat-developers-documentation-rhdh/pr-1071/

[themr0c]

/cherry-pick release-1.5

/cherry-pick release-1.6

[openshift-cherrypick-robot]

@themr0c: new pull request created: #1074

In response to this:

/cherry-pick release-1.5

/cherry-pick release-1.6

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[openshift-cherrypick-robot]

@themr0c: new pull request created: #1075

In response to this:

/cherry-pick release-1.5

/cherry-pick release-1.6

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[themr0c]

/cherry-pick release-1.4

[openshift-cherrypick-robot]

@themr0c: new pull request created: #1076

In response to this:

/cherry-pick release-1.4

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.