Update Install Guide for 4.2

[nathandyer]

This PR updates the install guide to reference all the changes users will now encounter as they step through an installation using Qubes 4.2. While most of these changes a fairly straightforward (for example, updating the locations of items inside the Qubes menu), a few of the changes I've made should be made explicit here:

• I've added a warning to the top of the prepare and install pages noting that not all packages are available in the repos, and that these instructions will not work until that happens. We should remove this once 1.0.0 is released and packages are available on yum.securedrop.org.
• Likewise, there are links to the Qubes 4.2.2-rc1 iso (mainly as a placeholder) that need to be updated to stable 4.2.2 once that is ready.
• I removed the Fedora 40 upgrade guide, and the references to it within; from here forward, no Workstation users will encounter a situation where they would need to manually upgrade to the Fedora 40 template.
• I've expanded the docs in a few small ways, such as noting that the "Unable to unlock 'TailsData'" warning is to be expected, and to make it explicit how to easily verify the Qubes ISO

Testing

• [ ] Visual review
• [ ] CI passes

[nathandyer]

Alright, I think I've addressed everyone's feedback. If I've overlooked anything, please let me know. A rough list of changes I made:

• Removed the warning and all language about pre-release/unstable software
• Updated Qubes references to point to 4.2.2 (not rc), with the knowledge we can fall back to 4.2.1 if need be
• Added in the additional changes from Erik's draft PR in #222
• Added a commit to remove the section about rotating legacy passphrases (we can drop the commit if we want to leave that in for now)
• It seems there is still some debate about how to handle the "Settings gear" references. The unicode icon is not an option for this release, and I think like @legoktm I prefer the more concise option of just referring to the "Settings gear icon" within the path - but will be glad to change it to the more verbose option ("on the left sidebar"... etc.) if we prefer it.

[legoktm]

I'm going to walk through this via yum-qa/apt-qa and approve if it all goes well.

[nathandyer]

Thanks for taking the new instructions for such a thorough test drive @legoktm! Back to you.