add extend content

[wirano]

What does this PR change? What problem does it solve?

Introduces an extent content hook, which is useful when adding custom codes below .Content.

e.g., adding mermaid diagrams, as hugo doc described.

Now, we just need to create /layouts/partials/extend_content.html and add a snippet to it. There is no need to edit the theme that makes the submodule dirty.

Compared to adding a snippet to extend_footer.html, this hook only affects post pages that have mermaid code blocks, which may speed up loading.

P.S., I don't know what the name of this hook should be. Maybe there is a better choice.

Was the change discussed in an issue or in the Discussions before?

#763 #850 #296

PR Checklist

• This change adds/updates translations and I have used the template present here.
• I have enabled maintainer edits for this PR.
• I have verified that the code works as described/as intended.
• This change adds a Social Icon which has a permissive license to use it.
• This change does not include any CDN resources/links.
• This change does not include any unrelated scripts such as bash and python scripts.
• This change updates the overridden internal templates from HUGO's repository.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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