Add multimodality table to documentation

[darrylong]

Description

This PR adds an additional table below the existing model table in readthedocs, showing the modality each model supports.
Also, layout of page is widened to support more content in tables.

An preview of the table is as follows:

Related Issues

Fixes #634

Checklist:

• I have added tests.
• I have updated the documentation accordingly.
• I have updated README.md (if you are adding a new model).
• I have updated examples/README.md (if you are adding a new example).
• I have updated datasets/README.md (if you are adding a new dataset).

[tqtg]

Couple of things:

1. Too many red crosses (unappealing), we might just leave them as blank.
2. Let's not have the Feature column. For any model that support FeatureModality, we can tick all the corresponding user/item Text, Image, and Graph because these modalities can be transformed to features. This will simplify the table too.

Let me know what you think.

[darrylong]

Couple of things:

1. Too many red crosses (unappealing), we might just leave them as blank.
2. Let's not have the Feature column. For any model that support FeatureModality, we can tick all the corresponding user/item Text, Image, and Graph because these modalities can be transformed to features. This will simplify the table too.

Let me know what you think.

@lthoang let me know what you think as well. thanks

[darrylong]

This is how it looks like after removing user/item feature columns and removing the crosses.

[lthoang]

This is how it looks like after removing user/item feature columns and removing the crosses.

I agree that this one looks better.

[tqtg]

Yeah, it looks great. One minor thing, it might be better to add column dividers/separators for the table.

[darrylong]

This is the update after adding dividers and removing crosses.

[tqtg]

LGTM!