[ENH] A simple {tableofcontents} directive

[AakashGfude]

Have transferred the PR here from MyST-NB executablebooks/MyST-NB#167 along with a few implementations of suggestions by @choldgraf

Syntax :-

```{tableofcontents}

At present it lays out the sub pages of the page where it is declared. I think we might also want it to have internal section links of the page as bullet points. Is that what you meant as well with your suggestion @choldgraf here - executablebooks/MyST-NB#167? or just the external sub-pages.

Adding it in the intro.md page gives :-

Adding it to /content-types/index.md page gives :-

[AakashGfude]

Possible second phase features :-

• passing of a relative path to another page as an argument, and if so then show the toctree for that page.

[choldgraf]

@AakashGfude is this ready to go? (maybe try rebasing on master to see if the tests pass afterward? or fix those tests if we're ready to merge)

[AakashGfude]

@choldgraf do the functionalities look good enough to be merged for now? If yes, then I will take out some time, and correct the tests and also push the styles in sphinx-book-theme.
We can add other changes later then.
Note that it does not give the links to the internal sections of the page for now, which I was not sure should be implemented or not in this directive.

[choldgraf]

I think it's a good first step! Let's get it merged and try it out to see how it feels

[AakashGfude]

@choldgraf do you know why the test https://circleci.com/gh/executablebooks/jupyter-book/303?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link is failing?
It says No module named 'jupyter_book.directive'. It seems like the error you get when it is using an old installation of jupyter_book locally for tests.

[choldgraf]

I think the problem could be that you created a module for the directive but you didn’t add an init.py file

[AakashGfude]

Thanks @choldgraf . The tests seem to be passing now.

[choldgraf]

oops wait a sec - I just realized you didn't add any tests...can you please add a test to make sure that the structure is what we'd expect? I think just a simple pytest-regressions check should be fine.

[AakashGfude]

@choldgraf hmm. for the toctree directive though, I will need a project with toc.yml and a page with the directive used. Should I create it in the examples folder?

[choldgraf]

Looking good! A few quick comments

[choldgraf]

re-check the toc.py page - the behavior just changed a little bit, mostly so that page titles are only added with a flag and not by default

[AakashGfude]

@choldgraf have done the necessary changes. Let me know what you think

[choldgraf]

it looks great - let's give it a merge and see how the functionality feels! thanks @AakashGfude 🎉