Added a menuTitle attribute to partials/menu.html

[mreithub]

Right now the title frontmatter argument is used both for the page title and for its entry in the menu hierarchy.

This makes it hard to use descriptive titles. Take the following hierarchy for example:

• Install
  • Linux
  • ...

The Linux page would have to use that exact title. A better (and more descriptive) page title would be Install on Linux (but that's a little long for the menu).

This patch will use the official linkTitle if present.
And if the custom menuTitle is defined, that one will be used instead.

[matcornic]

Thank you very much for this PR, I totally agree with the needs :). 2 questions though:

• What would be the point of using menuTitle instead of linkTitle, expressiveness ?
• Could you update the doc as well with this new feature ? https://learn.netlify.com/en/cont/pages/#front-matter-configuration (https://github.com/matcornic/hugo-theme-learn/blob/master/exampleSite/content/cont/pages/_index.en.md). If you don't speak french, I will translate.

[mreithub]

I wasn't entirely sure if linkTitle was totally intended for that purpose, so I thought I'd give people a (yet another) choice. But I'm totally ok with not including menuTitle (I figured it would be easier to explain if someone asks about it :) )

I'll update the doc (I don't speak french though).

[matcornic]

No problem, keep menuTitle as it’s more expressive but add an explanation in the doc 👌
Thanks 🙏 !