Various improvements to the website.

[quyykk]

This PR changes the following:

• Adds the jekyll generated site to .gitignore
• Adds a new panel on the sidebar that links to Github, Steam and the Discord.
• The Downloads link now points to the latest release instead of the general releases page.
• Adds a helpful website to generate a plugin manifest file for the repo (courtesy of @MCOfficer, all credit goes to him)
• Fixes the render thumbnails being drawn over the fullscreen gallery image.
• Removed all the irrelevant text on the plugins page (I plan on adding that information on the ES Wiki for creating plugins)
• Changed the default amount of plugins shown to 50.
• Added two sentences to the plugins page that links to the ES Wiki on how to add your plugin to the repo (I will publish that page if this PR gets merged)
• Added a title to the plugins page.
• Changed the Forums link to the Github Discussions link (since the Google Forums are dead).

That's it!

Screenshots

[Amazinite]

Looks nice.

[tehhowch]

I didn't spin anything up locally, just a ultra-quick glance at the new page.

[tehhowch]

No global variables. Use let or const to declare it.

[quyykk]

I added a let licenses = [];. Does that fix it? I'm a total beginner in JS.

[jwmh]

?
Public website at https://endless-sky.github.io/ still lists inaccessible-to-newcomers GoogleGroups link
see endless-sky/endless-sky#5743 (comment)

Zitchas commented on Jun 5, 2021
...
For accounts that have NOT visited it before:

• 0 [people said] : I can view the group while signed into an account that has never visited it before.
• 48 [people said] : I can NOT view the group while signed into an account that has never visited it before.

... and the latter is true for myself, as well.
As a newcomer,
I CANNOT visit the GoogleGroups link from the website.

Please remove it!
(Worry about adding other more appropriate links later.)

As my token addition...
I also fully agree w a dev's point:
It is actively harmful to the "optics" of this project for [newcomer] folks to see an inaccessible Forums link.

... Having said that,
"Be careful what you wish for."

i.e.,
As other website-admins have noted...
... Sometimes, more convenience creates attracts lower-quality comments.
... which are an even bigger pain to administer / moderate!

P.S.
Apologies; I assume this has simply been overlooked as part of volunteer-devs' time who are (much more importantly!) working on improving the game itself ;)

I really appreciate all y'all!
You're great.
Seriously.

[Zitchas]

I can't speak to the code; but the proposed changes look like good ones.