docs: consistent MSRV docs & policy explanation

[hawkw]

Motivation

PR #934 fixed a bug in the CI configuration where MSRV checks were not
being run correctly. After this was fixed, it was necessary to bump the
MSRV to 1.40.0, as the tests were no longer actually passing on 1.39,
because some dependencies no longer support it.

While updating the documentation to indicate that the new MSRV is 1.40,
I noticed that the note on the MSRV was located inconsistently in the
READMEs and lib.rs documentation of various crates, and missing
entirely in some cases. Additionally, there have been some questions on
what our MSRV policies are, and whether MSRV bumps are considered
breaking changes (see e.g. #936).

Solution

I've updated all the MSRV notes in the documentation and READMEs to
indicate that the MSRV is 1.40. I've also ensured that the MSRV note is
in the same place for every crate (at the end of the "Overview" section
in the docs), and that it's formatted consistently.

Furthermore, I added a new section to the READMEs and lib.rs docs
explaining the current MSRV policy in some detail. Hopefully, this
should answer questions like #936 in the future. The MSRV note in the
overview section includes a link to the section with further details.

Finally, while doing this, I noticed a couple of crates
(tracing-journald and tracing-serde) were missing top-level lib.rs
docs. Rather than just adding an MSRV note and nothing else, I went
ahead and fixed this using documentation from those crate's READMEs.