docs: update custom extractor tutorial

[jared-rimmer]

I have ported over and merged content from:

https://meltano.com/blog/how-to-build-a-custom-extractor-with-meltano/

with the existing custom extractor tutorial in an attempt to making it a one stop shop for people wanting to get started with a creating a custom extractor.

Open questions

• Terminology: I've used "custom extractor" throughout and changed references from tap, plugin, project, source. What are your thoughts on this?
• SEO: As a lot of the content is a literal copy and paste from the blog it might have implications of duplicate content. Would you prefer a more thorough rewrite?

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Name	Link
🔨 Latest commit	4e3b7f2

[codecov]

Codecov Report

Merging #6703 (4e3b7f2) into main (7bbc90b) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #6703      +/-   ##
==========================================
+ Coverage   88.68%   88.70%   +0.01%     
==========================================
  Files         283      283              
  Lines       20401    20401              
  Branches     2010     2010              
==========================================
+ Hits        18093    18096       +3     
+ Misses       1971     1968       -3     
  Partials      337      337              

Impacted Files	Coverage Δ
src/meltano/core/plugin_discovery_service.py	88.75% <0.00%> (-0.60%)	⬇️
src/meltano/core/plugin_location_remove.py	90.81% <0.00%> (+4.08%)	⬆️

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[tayloramurphy]

@afolson you know more about SEO than me - what's our preference on this? cc @lauraperezco

[lauraperezco]

@afolson you know more about SEO than me - what's our preference on this? cc @lauraperezco

@tayloramurphy same as the ELT page applies here, the main website has preference over subpages for SEO, so Jared's assesment is correct! I think ideally we should replace the current content in docs with a short introduction that takes you to the blog link for the full tutorial (and maybe reference some other blog-tutorials in the future to make this section in docs more complete while giving more authority to our website)

[jared-rimmer]

@lauraperezco happy to update with your suggestions 👍

From the Slack thread there was an aim to make the docs the place where users can find all this type of information. Linking out to blogs still achieves this

[sbalnojan]

Hey @jared-rimmer, no it's not necessary to update the page to include less. It can stay as you suggested, everything in once place.

But FWIW we're aware that we need to figure out a guide for what goes where, that's still WIP ;)

[sbalnojan]

all changes adressed!

PR creator addressed all changes.