Change: Rename the Manual navigation button to Wiki

[J-PIE-314]

Before this PR:
Clicking the Manual button on the navigation bar would lead to OpenTTD's wiki.

This PR:
The Manual button is now renamed to Wiki which it already leads to. The link on the right advertising the manual remains.

[2TallTyler]

I disagree with this change, personally. The wiki homepage has a large link to the Manual, while the Community page only has a small link to TT-Forums. This feels like moving stuff around with no real benefit — and doesn't fix the truly excessive number of main menu buttons (I guess the website is like our game UI). 😄

[J-PIE-314]

How about we change the Manual button to instead say Wiki? That is what it leads to and there is already a button for people want to know how to play.

[auge8472]

Even the OpenTTD-Wiki is not only the game play manual itself, an additional link will IMHO not clarify anything. I expect it to be more confusing for some people because of the two links. IMHO the one currently existing link to the selection page (game play manual, development manual, addon development manual, (community)) should remain.

Also I would not like the removal of the direct link to the forum but that's debatable because of the more frequented other channels (especially the Discord chat). Even though I myself use the forum almost exclusively, you can also argue that a lot more people use the chats instead.

[J-PIE-314]

I scaled back this PR to only focus on the Manual button.