Document `GIT_STATUS_OPT_INCLUDE_UNREADABLE`, and some minor cleanups.

Consider files executable only if the user can execute them

Update from regex to pcre licensing information in docs/contributing.md

docs: fix some missing includes that cause Docurium to error out

Define WINHTTP_NO_CLIENT_CERT_CONTEXT if needed

docs: stop mentioning libgit2's "master" branch

Co-authored-by: lhchavez <lhchavez@lhchavez.com>

The name of libgit2's main branch is now "main" but contributing.md and
release.md still reference "master" as the main branch.  Fix it.

This is what git.git does, so we should follow suit.

Tolerate readlink size less than st_size

Patch documentation

In https://github.com/libgit2/libgit2/pull/5723/files/fc46dc06f52f854f74371682f911f13856c68edb#r540092847, I was confused by the semantics of `git_index_find`. The documentation says both that it returns the output value in `at_pos` and that it returns it directly as an `int`. This is incorrect; the return value is only returned via `at_pos`.

tests: clean up memory leak, fail on leak for win32

Fix issues with Proxy Authentication after httpclient refactor

Fix documentation formatting

Update README.md for additional Delphi bindings

Manually merging #5842