Improves ergonomics when pkg-config is missing and updates docs

[sgg]

pkg-config is needed to build hsbench and hscollider as it's used to dynamically link sqlite and libpcre however this is not outlined by the build process nor in the dev docs (outlined in #135).

This PR includes two changes:

1. I've updated the CMake files to print an explicit log message when pkg-config is missing
2. I've added a section to the Getting Started section of the docs that outlines the optional dependencies and what they are used for.

---

Before

$ skathi ~/w/sgg-hyperscan git:master ❯❯❯ cmake -G Ninja
...
-- Could NOT find PkgConfig (missing: PKG_CONFIG_EXECUTABLE)
-- PCRE version 8.41 or above not found
-- PCRE 8.41 or above not found
-- Looking for pthread.h
-- Looking for pthread.h - found
-- Looking for pthread_create
-- Looking for pthread_create - found
-- Found Threads: TRUE
-- looking for sqlite3 in source tree
--   no sqlite3 in source tree
-- sqlite3 not found, not building hsbench
-- PCRE 8.41 not found, not building hscollider

After

$ skathi ~/w/sgg-hyperscan git:dep-docs ❯❯❯ cmake -G Ninja
-- Could NOT find PkgConfig (missing: PKG_CONFIG_EXECUTABLE)
-- Could not find pkg-config. Will not generate pkg-config metadata for libhs.
...
-- Could NOT find PkgConfig (missing: PKG_CONFIG_EXECUTABLE)
-- pkg-config was not found. This is required to dynamically link libpcre.
-- PCRE 8.41 or above not found
-- Could NOT find PkgConfig (missing: PKG_CONFIG_EXECUTABLE)
-- pkg-config was not found. This is required to dynamically link sqlite3.
-- looking for sqlite3 in source tree
--   no sqlite3 in source tree
-- sqlite3 not found, not building hsbench
-- PCRE 8.41 not found, not building hscollider