Improve manpage format

[straight-shoota]

• Use title case for headlines (automatically transformed to uppercase in manpage transform)
• Uniformly use bold text for code highlights (backticks do not work)

These improvements affect both the manpage format and enable a decent HTML rendering (see https://github.com/crystal-lang/crystal-book/issues/476).

[oprypin]

Regarding "backticks do not work" - I see that that is not represented in any way when viewing the manpage on the terminal. OK.

I don't think it's good to change all underlines to bold, because underlines seem to have a very specific meaning - some kind of literal sample -- actually very close to what backticks would've been used with. And the underline looks very good on man pages.

[straight-shoota]

I know, it's not an optimal result. If it was feasible, I would go for any improvement. But I think that's the best we can get for a reasonable effort. IMO the manpage representation looks reasonable and consistenst.