Fix release announcements.

[grembo]

Summary

• Sidenav download would link to 12.1 instead of 12.2.
• Template variables in various announcements weren't used correctly, which rendered the instructions unusable and broke links.
• dd instructions in release announcements didn't substitute release versions.
• Links in some release announcements used "r" suffic instead of "R" suffix in links.

Example:
Go to https://www.freebsd.org/releases/12.1R/announce/ and try to:

• jump to 12.2 (which won't work).
• see links to 12.1 resources (which contain quotes "), which breaks them.
• look at instructions (including "dd" examples), which doesn't have the template variables replaced (copy and pasting this won't make users happy)
• See extra quotes in various places.

Go to https://www.freebsd.org/releases/10.4R/announce/ - scroll down and test links to release notes/errata.

Test Plan

Tested locally (using latest gohugo from ports).

[grembo]

I'll make it build locally first and then re-open...

[grembo]

Fix things after incorporating changes that happened in the meantime.

[grembo]

@carlavilla This builds now locally, so it can be properly reviewed.

[carlavilla]

Approved. And commit done. Thanks for that.

[grembo]

In D28354#634399, @carlavilla wrote:

Approved. And commit done. Thanks for that.

Thanks!