Support to manual configure the content max width for Gemini Scheme.

[tsanie]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines.
• Tests for the changes have been added (for bug fixes / features).
  • Muse | Mist have been tested.
  • Pisces | Gemini have been tested.
• Docs have been added / updated (for bug fixes / features).

PR Type

What kind of change does this PR introduce?

• Bugfix.
• Feature.
• Code style update (formatting, local variables).
• Refactoring (no functional changes, no api changes).
• Build related changes.
• CI related changes.
• Documentation content changes.
• Other... Please describe:

What is the current behavior?

The content width is growing along with the screen width in the Gemini Scheme, some people want to give a max-width of it.

Issue Number(s): #102

What is the new behavior?

The user could set the content max-width by changing the settings in _config.yml, only available for Gemini Scheme and don't affect the other schemes.

• Screens with this changes: N/A
• Link to demo site with this changes: https://tsanie.us/ (with a max-width of 1000px)

How to use?

In NexT _config.yml:

# Manual define the max content width
# !!Only available for Gemini Scheme currently
# Leave it empty for the default 75% (suggest not less than 1000px)
max_content_width: 1000px # or 90% for more wide, etc.

Does this PR introduce a breaking change?

• Yes.
• No.

[ivan-nginx]

If for Gemini only, need to add this in Gemini scheme file maybe?

[tsanie]

Yeah, maybe like the below?