If a markdown page in docs/ is an alias of another page, such as

    --8<-- "components/mbedtls/README.md"

then the plugin will keep a record of the actual source markdown's path,
which is `components/mbedtls/README.md` in this example. Later on, when
converting file links in the output HTML file, the plugin will treat
every link as relative to the actual source markdown instead of the
currently processed page.

Additionally, this commits adds a validation that each local file link
points to path that actually exists in the local repository. If the link
destination is non-existent, the plugin raises an exception in order to
notify the documentation author of the link error. Note that MkDocs's
built-in strict mode also enforces such checks, but it cannot handle
links in alias pages, so we have to disable the strict mode and let our
plugin do the checks instead.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>