Better GitHub issue templates

[jessicaschilling]

This issue is part of Epic 3A: Fixes from user feedback/other metrics.

Add friendly templating for new GH issues in /docs to make it easier for folks to articulate docs problems they may find, and for us to get the required info we need to fix them.

Consider adding several -- e.g. for errors/broken links vs more nuanced content-based issues?

[jessicaschilling]

Just realized I'm not a maintainer on my own team's repo 😆 ... last time we made changes to GitHub issue templates I had to ask @Stebalien to do it. Actually -- @Stebalien, would you be willing to add me and @cwaring as maintainers? Otherwise, I can ask you to add another issue template for me.

[jessicaschilling]

(Notes to self, or whoever makes the changes:)
For this repo's existing issue template ...

1. Please change the title from "Issue" to "Documentation Issue"
2. Please change the subtitle from "Report a documentation issue" to "Report a problem with existing documentation"
3. Please add the label OKR 3: Content Improvement
4. Please insert the following prepopulated text in the body of the issue:

Did you find a bug, typo or other issue in an item of IPFS documentation? Please use this template to let us know!

**URL of the page in question:**
_e.g. https://docs.ipfs.io/introduction/usage/_

**What's wrong with this page?**
_Please be as specific as possible. If relevant, screenshots are helpful!_

**What would you like to have changed/amended/fixed in order to make things right?**
_Again, the more specific you are, the easier/faster fixing this item can be._

**Would you be interested in helping fix this page?**
_If yes, that would be wonderful! Feel free to create a PR against this new issue, and it'll get reviewed right away._

**Is there any other feedback you'd like to share about IPFS docs?**

[jessicaschilling]

(Notes to self, or whoever makes the changes:)
Please create a new issue template in the docs repo ...

1. Title: "Content Request"
2. Subtitle: "Suggest a new item for the IPFS docs"
3. Please add the labels docs-ipfs, OKR 3: Content Improvement and design-content
4. Please insert the following prepopulated text in the body of the issue:

Is there an item missing from IPFS documentation that would be helpful to you? Please use this template to let us know!

**What's the subject of the item you're requesting?**
_In other words, what would the title of this new page be?_

**What's the content of the item you're requesting?**
_Please describe the new page's content in more detail, including any ideas for the APIs or methods
to be taught, specific details to be addressed, or real-world examples to be included._

**What will the user know how to do after going through this item?**
_This might be the same as the title of the page you're proposing! If not, please add details to help us understand._

**Would you be interested in helping create this item?**
_If yes, that would be wonderful! Feel free to create a PR against this new issue, and it'll get reviewed right away. Or if you'd like to brainstorm with other contributors and work in collaboration to create this content, that's great too; just say so._

**Is there any other feedback you'd like to share about IPFS docs?**

[jessicaschilling]

Made changes noted in comments above - we now have two new issue templates for content requests and documentation issues.

This broke the "open a GitHub issue" link in the docs site near the hot-or-not buttons, so fixed that in #359.

Calling this one good!