Documentation fixes

ci: add an individual coverity pipeline

Coverity is back but it's only read-only!  Agh.  Just allow it to fail
and not impact the overall job run.

ci: run docurium to create documentation

ci: return coverity to the nightlies

Clean up some warnings

Nightlies: use `latest` docker images

Run docurium as part of the build.  The goal of this is to be able to
evaluate the documentation in a given pull request; as such, this does
not implement any sort of deployment pipeline.

This will allow us to download a snapshot of the documentation from the
CI build and evaluate the docs for a particular pull request; before
it's been merged.

Validate that the return value of the read is not less than INT_MAX,
then cast.

Index entries are 32 bit unsigned ints, not `size_t`s.

The git_describe_format_options.abbreviated_size type is an unsigned
int.  There's no need for it to be anything else; keep it what it is.

Quiet down a warning from MSVC about how we're potentially losing data.
Validate that our data will fit into the type provided then cast.

The transport code returns an `int` with the number of bytes written;
thus only attempt to write at most `INT_MAX`.

Windows doesn't include ssize_t or its _MAX value by default.  We are
already declaring ssize_t as SSIZE_T, which is __int64_t on Win64 and
long otherwise.  Include its _MAX value as a correspondence to its type.

Our streams implementation takes a `size_t` that indicates the length of
the data buffer to be written, and returns an `ssize_t` that indicates
the length that _was_ written.  Clearly no such implementation can write
more than `SSIZE_MAX` bytes.  Ensure that each TLS stream implementation
does not try to write more than `SSIZE_MAX` bytes (or smaller; if the
given implementation takes a smaller size).

Quiet down a warning from MSVC about how we're potentially losing data.
This is safe since we've explicitly tested it.

A number of source files have their implementation #ifdef'd out (because
they target another platform).  MSVC warns on empty compilation units
(with warning LNK4221).  Ignore warning 4221 when creating the object
library.

Cast actual filesystem data to the int32_t that index entries store.

The filesystem iterator takes `stat` data from disk and puts them into
index entries, which use 32 bit ints for time (the seconds portion) and
filesize.  However, on most systems these are not 32 bit, thus will
typically invoke a warning.

Most users ignore these fields entirely.  Diff and checkout code do use
the values, however only for the cache to determine if they should check
file modification.  Thus, this is not a critical error (and will cause a
hash recomputation at worst).

Our blob size is a `git_off_t`, which is a signed 64 bit int.  This may
be erroneously negative or larger than `SIZE_MAX`.  Ensure that the blob
size fits into a `size_t` before casting.

Quiet down a warning from MSVC about how we're potentially losing data.
Ensure that we're within a uint16_t before we do.

Quiet down a warning from MSVC about how we're potentially losing data.
This is safe since we've explicitly tested that it's positive and less
than SIZE_MAX.

Quiet down a warning from MSVC about how we're potentially losing data.
This is safe since we've explicitly tested that it's within the range of
0-100.

Quiet down a warning from MSVC about how we're potentially losing data.
This cast is safe since we've explicitly tested that `strip_len` <=
`last_len`.

Quiet down a warning from MSVC about how we're potentially losing data.

index: preserve extension parsing errors

Deprecate functions and constants more gently

Ensure that we do not use any deprecated functions in the library
source, test code or examples.

Users can define `GIT_DEPRECATE_HARD` if they want to remove all
functions that we've "softly" deprecated.

Move the deprecated stream tests into their own compilation unit.  This
will allow us to disable any preprocessor directives that apply to
deprecation just for these tests (eg, disabling `GIT_DEPRECATED_HARD`).