update conf.py - changed linkcheck builder options to reduce [broken] false positives

[Argen1sw]

Related
#4209

Changes

• Defined linkcheck_anchors_ignore_for_url with a list of URLs where anchor validity will not be checked to avoid broken link and 'Anchor not found' error.
• Defined linkcheck_ignore with URLs of the local links that were returning broken by checklink builder (Took it from #4487 )
• Added linkcheck_request_headers with a default header that will be use for all the request that linkcheck builder will send to the links. This will help in reducing false positives caused by missing user-agent strings (403 Forbidden responses). Related thread

[Argen1sw]

Hi @fujitatomoya, as you are the author of issue #4209 I was wondering if you could review this pull request that will fix having too many false positives for make linkcheck.
Also as this is officially my first PR in this repo, I would appreciate any feedback you could have regarding the way I did it.
Thanks!

[fujitatomoya]

@Argen1sw

1st, thank you very much for the PR, appreciate it.
this is on my list, but i do not seem to have enough bandwidth right now, sorry. but i will take a look when i got some time.