Update SNAP Installation instructions

[osmosBe]

Since I struggled a bit installing gitea with the snap package and finally managed to get it right I thought I would give something back to this project.
So I created a little guide and thought I add it to your Documentation.
I hope it can be useful for others and I hope it was the right way to get the snap working.

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[techknowlogick]

I'm unsure about this PR, the thing about the snap is that it is not dependent on anything else and can be just installed and is ready to go.

If users want something more advanced then other options are available to them.

[wxiaoguang]

Maybe this snap method can be mereged into the package manager page, it is simple enough, while other steps are all covered by existing topics.

[osmosBe]

I'm unsure about this PR, the thing about the snap is that it is not dependent on anything else and can be just installed and is ready to go.

If users want something more advanced then other options are available to them.

The Problem with the snap instructions was, that I thought I could just install since there was no Information about the default port and that I needed an extra Database Setup etc.
Because most snap installations I did in the past where quite easy to setup and get running with the database and the config included or with some information on how to get it up and Running.
Maybe it is enough to reference the other topics or have another Page like Quickstart or Quicksetup where an overview of the needed components is given.
I know it's not that hard to use the snap and get it running but since the claim of this Project is: `The goal of this project is to provide the easiest, fastest, and most painless way of setting up a self-hosted Git service'. I think such a simple guide to get started is really usefull.

[axifive]

The Problem with the snap instructions was, that I thought I could just install since there was no Information about the default port and that I needed an extra Database Setup etc.

for sqlite not need extra Database Setup

[wxiaoguang]

Snap installation has more issues, which should be fixed or documented.

• Gitea Snap update issue #16209
  • Gitea Snap update issue #16209 #17157

[bagasme]

We have documentation for database setup and reverse proxy, so instead of duplicating the instructions you can simply point to those links.

[wxiaoguang]

Thank you for your contribution.

After reading the document again, I found that most contents were already covered by existing document:

• Database: https://docs.gitea.io/en-us/database-prep/
• HTTPS: https://docs.gitea.io/en-us/https-setup/
• Proxy: https://docs.gitea.io/en-us/reverse-proxies/
• Snap: https://docs.gitea.io/en-us/install-from-package/

So this PR can be closed.