Add generic home page section generation

[MarekPikula]

This change enables theme user to specify arbitrary order of sections, and add multiple sections of the same type (more generic approach than in #105, which is applicable only to experience blocks).

• All sections are specified in config as a list, with order of items directly reflecting order on the main page (including navbar).
• Type of a section is specified with a new type field.
• If there are multiple sections of the same type, in order to distinguish them for navbar navigation, you need to specify unique id field.
• Navbar and main page generation is more generic, allowing to add custom section types – the only requirement is a custom partial named after the section type. Styling can be provided in a custom CSS (enabled with customCSS site param).

[gurusabarish]

Hi @MarekPikula , Thanks for all your valuable pull requests ❤️ . All your pull requests can make this template to use in more effective way.

[gurusabarish]

As it involves more changes, I will check this pull request clearly and merge.

[MarekPikula]

Sure thing. The main issue I have with this PR in particular is that it introduces braking changes. I thought about making it backwards-compatible, but couldn't find a way to do it cleanly (yet). I'd say that we have a few options:

1. Try to make this change backwards-compatible. It may make the code look somewhat uglier, but it won't break people's setup.
2. Merge as is but provide a clear instructions on how to transition to the new config scheme. The question is where is the right place for this. I'd say that maybe a wiki article, a release pointing to that article, and maybe a note in the README.

What do you think?

[MarekPikula]

I just rebased the branch onto the recent master, so that it's directly mergable.