Indexing logic improvements

[karol-t-wilk]

Sorry for the late and big PR, I will make sure the next ones are more manageable 😅

There are several improvements to the architecture of the app:

• Multiple embedding models can be used at the same time
• The app is now aware of packages and their releases
• The app has support for future fragmentation of documentation
• Adding a package to the app and embedding its docs are now separate actions, implemented as mix tasks
• The demo page now lets you choose the embedding model used to perform the search
• The Hex client uses only repo.hex.pm and does not touch the REST API

[wojtekmach]

Looks good to me!

[wojtekmach]

this adds some strictness but I don't think it's worth it and so I'd just remove it.

Suggested change

	else
	{:error, _} = err ->
	err

[wojtekmach]

this clause doesn't do anything, why do we keep it?

[wojtekmach]

ditto:

Suggested change

	else
	{:error, _} = err ->
	err

[wojtekmach]

nitpick: this doesn't matter so I'm ok with keeping this as is but in practice we'd have the same package name but different version so I'd consider changing this and similar tests to be slightly more realistic:

Suggested change

	releases = [
	%Release{package_name: "lesser", version: Version.parse!("1.2.3")},
	%Release{package_name: "greater", version: Version.parse!("2.1.3")},
	%Release{package_name: "middle", version: Version.parse!("1.4.3")}
	]
	assert %Release{package_name: "greater"} =
	Release.latest(releases)
	releases = [
	%Release{version: Version.parse!("1.2.3")},
	%Release{version: Version.parse!("2.1.3")},
	%Release{version: Version.parse!("1.4.3")}
	]
	assert Release.latest(releases).version == Version.parse!("2.1.3")