Closes #151: Show headings in the sidebar

[nblthree]

Summary
Closes #151
What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI/CSS related code, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

• Show headings
• Change the hash depending on the scroll position
• Make sidebar sections's collapsible

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[krmax44]

Make sidebar section collapsible (???? sidebar section is already collapsible when the screen size is small)

I think it means adding a hamburger menu style button to allow collapsing it even on a desktop

[nblthree]

I just think that there is no need to do that because it became collapsible when the window inner width is inferior to 768px

[egoist]

I mean something like this.

[nblthree]

Seems that the deployment fell when importing ./file.vue that's what I understood from the last commit

[krmax44]

Pretty good 👍 Only some code style aspects that could be improved.

[krmax44]

So I have refactored some bits to make it cleaner and easier to understand and I tweaked some design aspects. Only remaining issue: when scrolling, the page jumps a bit because scrolling changes the hash and changing the hash changes the scroll position. So we need to decouple that.

[nblthree]

when scrolling, the page jumps

history.pushState(null, null, hash) will do the job with some other changes

You made some good changes but some I don't like I will wait until @egoist decide what he wants