Epic: Move website to custom domain https://macvim.org

[ychin]

Currently the canonical URL for MacVim's web page is https://macvim-dev.github.io/macvim/, since it's using the default GitHub Pages configuration. We have a domain (macvim.org) registered but it has just been doing a redirect forwarding to the above URL.

We should move to making https://macvim.org/ the canonical URL instead using GitHub Page's custom domain. It's easier to remember and less awkward to type, and looks more canonical as a website. Hosting it from the top-level domain also allows us to be more flexible in controlling it for the future, including if we want to move the website off from GitHub Pages instead of being tied to a particular service.

Also see #48 which set up the GitHub pages.

• [x] Move the appcast URL from a hard-coded GitHub raw URL to one under https://macvim.org (#1290). Note that this has the issue that old MacVim builds will still be using the older URL so we should keep the old gh-pages branch around and updated so the old appcast URL will keep working in the short-term future.
• [x] Set up a "macvim-dev.github.io" repo as an intermediate step so that MacVim's web page can be hosted as a organization GitHub page (https://macvim-dev.github.io) instead of a project one (the current https://macvim-dev.github.io/macvim). This makes sense for organization based around a single project, and has some neat advantages such as the trick to automatically have the docs page (https://github.com/macvim-dev/docs) work under it (https://macvim-dev.github.io/docs or the later https://macvim.org/docs). This is similar to what Neovim does with their website and a separate "doc" repo.
• [x] Protect and verify macvim.org for GitHub Pages usage for security (https://docs.github.com/en/pages/configuring-a-custom-domain-for-your-github-pages-site/verifying-your-custom-domain-for-github-pages)
• [x] Set up custom domain with CNAME pointing to it. https://github.com/macvim-dev/macvim-dev.github.io/commit/4baeaf501e1f88c84bec953cf564bf06420c2f87
• [x] Set up robots.txt and sitemap so that https://macvim.org/docs can be crawled by search engines. https://github.com/macvim-dev/macvim-dev.github.io/commit/ecd16adf110ea334e756946a44f82436cc32f1d5 and https://github.com/macvim-dev/docs/commit/55d0214b5900791d1e652c6ca09f971ab4fddda4
• [x] Set up redirect from the gh-pages branch so that https://macvim-dev.github.io/macvim (which will automatically redirect to https://macvim.org/macvim under the new custom domain) will redirect to just https://macvim.org. Note that this has to be meta-tag based as GitHub Pages doesn't support server-side redirect. https://github.com/macvim-dev/macvim/commit/54b911e075ace19756cbf06a0ee40d5006405920
• [x] Make sure Sparkle updates still work.

[ychin]

Should be all done now. Don't see any glaring issues but hopefully DNS propagates quickly enough that people using MacVim won't see an error when they try to check for updates.