Fix Readme links in Preferences screen

[lidel]

Github UI may be intimidating, we probably could use URLs from github pages to improve UX.

[jessicaschilling]

I'm thinking we may actually want to bring these links into IPFS docs themselves:

• We're pushing to place all explanatory material for our primary projects into one central location
• We're actually already linking to a bunch of pages in the IPFS docs in the prefs already

[lidel]

For reference, links to docs on Preferences are:

$ rg docs add-on/src/options

add-on/src/options/forms/file-import-form.js
22:                <p><a href="https://docs.ipfs.io/guides/concepts/mfs/" target="_blank">

add-on/src/options/forms/experiments-form.js
72:                <p><a href="https://github.com/ipfs-shipyard/ipfs-companion/blob/master/docs/x-ipfs-path-header.md#x-ipfs-path-header-support-in-ipfs-companion" target="_blank">
94:                <p><a href="https://github.com/ipfs-shipyard/ipfs-companion/blob/master/docs/window.ipfs.md#%EF%B8%8F-windowipfs-is-disabled-for-now" target="_blank">

add-on/src/options/forms/dnslink-form.js
28:                <p><a href="https://github.com/ipfs-shipyard/ipfs-companion/blob/master/docs/dnslink.md#dnslink-support-in-ipfs-companion" target="_blank">

add-on/src/options/forms/gateways-form.js
62:                  <p><a href="https://docs.ipfs.io/guides/guides/addressing/#subdomain-gateway" target="_blank">
121:                    <p><a href="https://docs.ipfs.io/guides/guides/addressing/#subdomain-gateway" target="_blank">

add-on/src/options/forms/ipfs-node-form.js
23:                <p><a href="https://github.com/ipfs-shipyard/ipfs-companion/blob/master/docs/node-types.md#node-types-in-ipfs-companion" target="_blank">

Some are bit long (eg. window.ipfs one), not sure if it all first on a single page, may have "Companion browser extension" category or something.

[jessicaschilling]

First part of this effort (getting the docs onto docs-beta.ipfs.io) is here: ipfs/ipfs-docs#182