feat: user configurable outDir

[Mando75]

Hi, this is my first time opening a PR in Vitepress, so please let me know if I need to change anything. I did my best to follow the contributing guidelines.

Changes

• Adds a new outDir option to the UserConfig in .vitepress/config.js.
  • User-provided outDir is resolved the same way as srcDir to keep things consistent.
  • The existing behavior is used as fallback if no outDir is provided by the user.
  • I chose to name it outDir instead of dest (from Vuepress) because the outDir option already existed on the internal SiteConfig type
• Updates the GitLab deployment docs to use the new outDir to publish files to the top-level public directory in accordance with GitLab pages deployment requirements
• Adds a .gitignore item for .idea, the .vscode equivalent for JetBrains IDE's (Webstorm, PHPStorm, etc.). I am a WebStorm user, and I figured it may make contributing easier for other developers using JetBrains products.
• Fixes a typo in a comment I noticed when making these changes.

Motivation

• There was an issue requesting this feature recently updated that shows there is interest. How to configure the build directory of vitepress ? #171
• A similar option exists in Vuepress
• This would make my life easier for deploying my Vitepress site to GitLab 😉

Notes

I included both the feature and doc changes in a single PR since I thought they should be updated together. If you would prefer them split into multiple PRs for better tracking of feat/doc changes, I can do that.

I looked for any tests I should update, but didn't find any. If I missed them, please let me know and I can add a test case.

I have tested it in my Vitepress project using npm link, and it works great. I recorded a video demo that I can provide if you would like to see a demo.

close #171