Docs and examples switching retains filter

[octopoulos]

For example, if you search for "fog" in the examples, then click on "docs" link, the filter is emptied and it shows "Create a scene" page.
Same thing if you search in docs and click on the examples link.

With this PR, the search is retained, which personally helps me a lot.
Typical scenario is:

• Let's see if there are examples of ... parametric, so I type "param", cool I see an example.
• Now let's see if there's any doc about it, so I click on the docs link, then: "arghh, I have to type it again!!"
  :)

Screenshots:

1. write "skin"
   

2. click on docs and this appears:
   

3. click again on examples and this appears:
   

[octopoulos]

I was using a sort of a non standard way to store the original href, with sectionLink.href2 = ... instead of using a dataset or setAttribute, which personally I don't mind doing at all, but after looking at the existing code, I realized the coding style was different, so I made some changes, and removed that href2 "hack", and instead I'm storing the original href value in a variable, that's it!

The functionality has not changed.

[mrdoob]

so I made some changes, and removed that href2 "hack"

Nice, I was going to mention that 🤓

[mrdoob]

For testing:
https://raw.githack.com/octopoulos/three.js/docex/examples/

[mrdoob]

Thanks!

[mrdoob]

We'll have to unify all that duplicated code from examples/index.html and docs/index.html some day...