docs: update template-route-mapping to user guide

[Happydong]

在使用自定义模板的过程中,重载主题配置花费了好久,所以在说明文档中补充提示说明

[vercel[bot]]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
halo-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Feb 26, 2024 9:02am

[f2c-ci-robot[bot]]

Adding the "do-not-merge/release-note-label-needed" label because no release-note block was detected, please follow our release note process to remove it.

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/test-infra repository.

[f2c-ci-robot[bot]]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Once this PR has been reviewed and has the lgtm label, please assign ruibaby for approval. For more information see the Kubernetes Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment Approvers can cancel approval by writing /approve cancel in a comment

[ruibaby]

需要同时修改 https://github.com/halo-dev/docs/blob/main/docs/developer-guide/theme/template-route-mapping.md